<P>Perl has three built-in data types: scalars, arrays of scalars, and
associative arrays of scalars, known as ``hashes''. Normal arrays
are ordered lists of scalars indexed by number, starting with 0 and with
negative subscripts counting from the end. Hashes are unordered
collections of scalar values indexed by their associated string key.</P>
<P>Values are usually referred to by name, or through a named reference.
The first character of the name tells you to what sort of data
structure it refers. The rest of the name tells you the particular
value to which it refers. Usually this name is a single <EM>identifier</EM>,
that is, a string beginning with a letter or underscore, and
containing letters, underscores, and digits. In some cases, it may
be a chain of identifiers, separated by <CODE>::</CODE> (or by the slightly
archaic <CODE>'</CODE>); all but the last are interpreted as names of packages,
to locate the namespace in which to look up the final identifier
(see <A HREF="../../lib/Pod/perlmod.html#packages">Packages in the perlmod manpage</A> for details). It's possible to substitute
for a simple identifier, an expression that produces a reference
to the value at runtime. This is described in more detail below
and in <A HREF="../../lib/Pod/perlref.html">the perlref manpage</A>.</P>
<P>Perl also has its own built-in variables whose names don't follow
these rules. They have strange names so they don't accidentally
collide with one of your normal variables. Strings that match
parenthesized parts of a regular expression are saved under names
containing only digits after the <CODE>$</CODE> (see <A HREF="../../lib/Pod/perlop.html">the perlop manpage</A> and <A HREF="../../lib/Pod/perlre.html">the perlre manpage</A>).
In addition, several special variables that provide windows into
the inner working of Perl have names containing punctuation characters
and control characters. These are documented in <A HREF="../../lib/Pod/perlvar.html">the perlvar manpage</A>.</P>
<P>Scalar values are always named with '$', even when referring to a
scalar that is part of an array or a hash. The '$' symbol works
semantically like the English word ``the'' in that it indicates a
single value is expected.</P>
<PRE>
$days # the simple scalar value "days"
$days[28] # the 29th element of array @days
$days{'Feb'} # the 'Feb' value from hash %days
$#days # the last index of array @days</PRE>
<P>Entire arrays (and slices of arrays and hashes) are denoted by '@',
which works much like the word ``these'' or ``those'' does in English,
in that it indicates multiple values are expected.</P>
<PRE>
@days # ($days[0], $days[1],... $days[n])
@days[3,4,5] # same as ($days[3],$days[4],$days[5])
@days{'a','c'} # same as ($days{'a'},$days{'c'})</PRE>
<P>Entire hashes are denoted by '%':</P>
<PRE>
%days # (key1, val1, key2, val2 ...)</PRE>
<P>In addition, subroutines are named with an initial '&', though this
is optional when unambiguous, just as the word ``do'' is often redundant
in English. Symbol table entries can be named with an initial '*',
but you don't really care about that yet (if ever :-).</P>
<P>Every variable type has its own namespace, as do several
non-variable identifiers. This means that you can, without fear
of conflict, use the same name for a scalar variable, an array, or
a hash--or, for that matter, for a filehandle, a directory handle, a
subroutine name, a format name, or a label. This means that $foo
and @foo are two different variables. It also means that <CODE>$foo[1]</CODE>
is a part of @foo, not a part of $foo. This may seem a bit weird,
but that's okay, because it is weird.</P>
<P>Because variable references always start with '$', '@', or '%', the
``reserved'' words aren't in fact reserved with respect to variable
names. They <EM>are</EM> reserved with respect to labels and filehandles,
however, which don't have an initial special character. You can't
have a filehandle named ``log'', for instance. Hint: you could say
<A HREF="../../lib/Pod/perlfunc.html#item_open"><CODE>open(LOG,'logfile')</CODE></A> rather than <A HREF="../../lib/Pod/perlfunc.html#item_open"><CODE>open(log,'logfile')</CODE></A>. Using
uppercase filehandles also improves readability and protects you
from conflict with future reserved words. Case <EM>is</EM> significant--``FOO'',
``Foo'', and ``foo'' are all different names. Names that start with a
letter or underscore may also contain digits and underscores.</P>
<P>It is possible to replace such an alphanumeric name with an expression
that returns a reference to the appropriate type. For a description
of this, see <A HREF="../../lib/Pod/perlref.html">the perlref manpage</A>.</P>
<P>Names that start with a digit may contain only more digits. Names
that do not start with a letter, underscore, or digit are limited to
one character, e.g., <CODE>$%</CODE> or <CODE>$$</CODE>. (Most of these one character names
have a predefined significance to Perl. For instance, <CODE>$$</CODE> is the
current process id.)</P>
<P>
<H2><A NAME="context">Context</A></H2>
<P>The interpretation of operations and values in Perl sometimes depends
on the requirements of the context around the operation or value.
There are two major contexts: list and scalar. Certain operations
return list values in contexts wanting a list, and scalar values
otherwise. If this is true of an operation it will be mentioned in
the documentation for that operation. In other words, Perl overloads
certain operations based on whether the expected return value is
singular or plural. Some words in English work this way, like ``fish''
and ``sheep''.</P>
<P>In a reciprocal fashion, an operation provides either a scalar or a
list context to each of its arguments. For example, if you say</P>
<PRE>
int( <STDIN> )</PRE>
<P>the integer operation provides scalar context for the <>
operator, which responds by reading one line from STDIN and passing it
back to the integer operation, which will then find the integer value
of that line and return that. If, on the other hand, you say</P>
<PRE>
sort( <STDIN> )</PRE>
<P>then the sort operation provides list context for <>, which
will proceed to read every line available up to the end of file, and
pass that list of lines back to the sort routine, which will then
sort those lines and return them as a list to whatever the context
of the sort was.</P>
<P>Assignment is a little bit special in that it uses its left argument
to determine the context for the right argument. Assignment to a
scalar evaluates the right-hand side in scalar context, while
assignment to an array or hash evaluates the righthand side in list
context. Assignment to a list (or slice, which is just a list
anyway) also evaluates the righthand side in list context.</P>
<P>When you use the <CODE>use warnings</CODE> pragma or Perl's <STRONG>-w</STRONG> command-line
option, you may see warnings
about useless uses of constants or functions in ``void context''.
Void context just means the value has been discarded, such as a
statement containing only <CODE>"fred";</CODE> or <A HREF="../../lib/Pod/perlfunc.html#item_getpwuid"><CODE>getpwuid(0);</CODE></A>. It still
counts as scalar context for functions that care whether or not
they're being called in list context.</P>
<P>User-defined subroutines may choose to care whether they are being
called in a void, scalar, or list context. Most subroutines do not
need to bother, though. That's because both scalars and lists are
automatically interpolated into lists. See <A HREF="../../lib/Pod/perlfunc.html#wantarray">wantarray in the perlfunc manpage</A>
for how you would dynamically discern your function's calling
<H2><A NAME="scalar value constructors">Scalar value constructors</A></H2>
<P>Numeric literals are specified in any of the following floating point or
integer formats:</P>
<PRE>
12345
12345.67
.23E-10 # a very small number
4_294_967_296 # underline for legibility
0xff # hex
0377 # octal
0b011011 # binary</PRE>
<P>String literals are usually delimited by either single or double
quotes. They work much like quotes in the standard Unix shells:
double-quoted string literals are subject to backslash and variable
substitution; single-quoted strings are not (except for <CODE>\'</CODE> and
<CODE>\\</CODE>). The usual C-style backslash rules apply for making
characters such as newline, tab, etc., as well as some more exotic
forms. See <A HREF="../../lib/Pod/perlop.html#quote and quotelike operators">Quote and Quote-like Operators in the perlop manpage</A> for a list.</P>
<P>Hexadecimal, octal, or binary, representations in string literals
(e.g. '0xff') are not automatically converted to their integer
representation. The <A HREF="../../lib/Pod/perlfunc.html#item_hex"><CODE>hex()</CODE></A> and <A HREF="../../lib/Pod/perlfunc.html#item_oct"><CODE>oct()</CODE></A> functions make these conversions
for you. See <A HREF="../../lib/Pod/perlfunc.html#hex">hex in the perlfunc manpage</A> and <A HREF="../../lib/Pod/perlfunc.html#oct">oct in the perlfunc manpage</A> for more details.</P>
<P>You can also embed newlines directly in your strings, i.e., they can end
on a different line than they begin. This is nice, but if you forget
your trailing quote, the error will not be reported until Perl finds
another line containing the quote character, which may be much further
on in the script. Variable substitution inside strings is limited to
scalar variables, arrays, and array or hash slices. (In other words,
names beginning with $ or @, followed by an optional bracketed
expression as a subscript.) The following code segment prints out ``The
price is $100.''</P>
<PRE>
$Price = '$100'; # not interpreted
print "The price is $Price.\n"; # interpreted</PRE>
<P>As in some shells, you can enclose the variable name in braces to
disambiguate it from following alphanumerics. You must also do
this when interpolating a variable into a string to separate the
variable name from a following double-colon or an apostrophe, since
these would be otherwise treated as a package separator:</P>
print "We use ${who}speak when ${who}'s here.\n";</PRE>
<P>Without the braces, Perl would have looked for a $whospeak, a
<CODE>$who::0</CODE>, and a <CODE>$who's</CODE> variable. The last two would be the
$0 and the $s variables in the (presumably) non-existent package
<CODE>who</CODE>.</P>
<P>In fact, an identifier within such curlies is forced to be a string,
as is any simple identifier within a hash subscript. Neither need
quoting. Our earlier example, <CODE>$days{'Feb'}</CODE> can be written as
<CODE>$days{Feb}</CODE> and the quotes will be assumed automatically. But
anything more complicated in the subscript will be interpreted as
an expression.</P>
<P>A literal of the form <CODE>v1.20.300.4000</CODE> is parsed as a string composed
of characters with the specified ordinals. This provides an alternative,
more readable way to construct strings, rather than use the somewhat less
readable interpolation form <CODE>"\x{1}\x{14}\x{12c}\x{fa0}"</CODE>. This is useful
for representing Unicode strings, and for comparing version ``numbers''
using the string comparison operators, <CODE>cmp</CODE>, <CODE>gt</CODE>, <CODE>lt</CODE> etc.
If there are two or more dots in the literal, the leading <CODE>v</CODE> may be
omitted.</P>
<PRE>
print v9786; # prints UTF-8 encoded SMILEY, "\x{263a}"
print v102.111.111; # prints "foo"
print 102.111.111; # same</PRE>
<P>Such literals are accepted by both <A HREF="../../lib/Pod/perlfunc.html#item_require"><CODE>require</CODE></A> and <A HREF="../../lib/Pod/perlfunc.html#item_use"><CODE>use</CODE></A> for
doing a version check. The <CODE>$^V</CODE> special variable also contains the
running Perl interpreter's version in this form. See <A HREF="../../lib/Pod/perlvar.html#$^v">$^V in the perlvar manpage</A>.</P>
<P>The special literals __FILE__, __LINE__, and __PACKAGE__
represent the current filename, line number, and package name at that
point in your program. They may be used only as separate tokens; they
will not be interpolated into strings. If there is no current package
(due to an empty <CODE>package;</CODE> directive), __PACKAGE__ is the undefined
value.</P>
<P>The two control characters ^D and ^Z, and the tokens __END__ and __DATA__
may be used to indicate the logical end of the script before the actual
end of file. Any following text is ignored.</P>
<P>Text after __DATA__ but may be read via the filehandle <CODE>PACKNAME::DATA</CODE>,
where <CODE>PACKNAME</CODE> is the package that was current when the __DATA__
token was encountered. The filehandle is left open pointing to the
contents after __DATA__. It is the program's responsibility to
<A HREF="../../lib/Pod/perlfunc.html#item_close"><CODE>close DATA</CODE></A> when it is done reading from it. For compatibility with
older scripts written before __DATA__ was introduced, __END__ behaves
like __DATA__ in the toplevel script (but not in files loaded with
<A HREF="../../lib/Pod/perlfunc.html#item_require"><CODE>require</CODE></A> or <A HREF="../../lib/Pod/perlfunc.html#item_do"><CODE>do</CODE></A>) and leaves the remaining contents of the
file accessible via <CODE>main::DATA</CODE>.</P>
<P>See <A HREF="../../lib/SelfLoader.html">the SelfLoader manpage</A> for more description of __DATA__, and
an example of its use. Note that you cannot read from the DATA
filehandle in a BEGIN block: the BEGIN block is executed as soon
as it is seen (during compilation), at which point the corresponding
__DATA__ (or __END__) token has not yet been seen.</P>
<P>A word that has no other interpretation in the grammar will
be treated as if it were a quoted string. These are known as
``barewords''. As with filehandles and labels, a bareword that consists
entirely of lowercase letters risks conflict with future reserved
words, and if you use the <CODE>use warnings</CODE> pragma or the <STRONG>-w</STRONG> switch,
Perl will warn you about any
such words. Some people may wish to outlaw barewords entirely. If you
say</P>
<PRE>
use strict 'subs';</PRE>
<P>then any bareword that would NOT be interpreted as a subroutine call
produces a compile-time error instead. The restriction lasts to the
end of the enclosing block. An inner block may countermand this
by saying <CODE>no strict 'subs'</CODE>.</P>
<P>Arrays and slices are interpolated into double-quoted strings
by joining the elements with the delimiter specified in the <CODE>$"</CODE>
variable (<CODE>$LIST_SEPARATOR</CODE> in English), space by default. The
following are equivalent:</P>
<PRE>
$temp = join($", @ARGV);
system "echo $temp";</PRE>
<PRE>
system "echo @ARGV";</PRE>
<P>Within search patterns (which also undergo double-quotish substitution)
there is an unfortunate ambiguity: Is <CODE>/$foo[bar]/</CODE> to be interpreted as
<CODE>/${foo}[bar]/</CODE> (where <CODE>[bar]</CODE> is a character class for the regular
expression) or as <CODE>/${foo[bar]}/</CODE> (where <CODE>[bar]</CODE> is the subscript to array
@foo)? If @foo doesn't otherwise exist, then it's obviously a
character class. If @foo exists, Perl takes a good guess about <CODE>[bar]</CODE>,
and is almost always right. If it does guess wrong, or if you're just
plain paranoid, you can force the correct interpretation with curly
braces as above.</P>
<P>A line-oriented form of quoting is based on the shell ``here-document''
syntax. Following a <CODE><<</CODE> you specify a string to terminate
the quoted material, and all lines following the current line down to
the terminating string are the value of the item. The terminating
string may be either an identifier (a word), or some quoted text. If
quoted, the type of quotes you use determines the treatment of the
text, just as in regular quoting. An unquoted identifier works like
double quotes. There must be no space between the <CODE><<</CODE> and
the identifier. (If you put a space it will be treated as a null
identifier, which is valid, and matches the first empty line.) The
terminating string must appear by itself (unquoted and with no
surrounding whitespace) on the terminating line.</P>
<PRE>
print <<EOF;
The price is $Price.
EOF</PRE>
<PRE>
print <<"EOF"; # same as above
The price is $Price.
EOF</PRE>
<PRE>
print <<`EOC`; # execute commands
echo hi there
echo lo there
EOC</PRE>
<PRE>
print <<"foo", <<"bar"; # you can stack them
I said foo.
foo
I said bar.
bar</PRE>
<PRE>
myfunc(<<"THIS", 23, <<'THAT');
Here's a line
or two.
THIS
and here's another.
THAT</PRE>
<P>Just don't forget that you have to put a semicolon on the end
to finish the statement, as Perl doesn't know you're not going to
try to do this:</P>
<PRE>
print <<ABC
179231
ABC
+ 20;</PRE>
<P>If you want your here-docs to be indented with the
rest of the code, you'll need to remove leading whitespace
from each line manually:</P>
<PRE>
($quote = <<'FINIS') =~ s/^\s+//gm;
The Road goes ever on and on,
down from the door where it began.
FINIS</PRE>
<P>
<H2><A NAME="list value constructors">List value constructors</A></H2>
<P>List values are denoted by separating individual values by commas
(and enclosing the list in parentheses where precedence requires it):</P>
<PRE>
(LIST)</PRE>
<P>In a context not requiring a list value, the value of what appears
to be a list literal is simply the value of the final element, as
with the C comma operator. For example,</P>
<PRE>
@foo = ('cc', '-E', $bar);</PRE>
<P>assigns the entire list value to array @foo, but</P>
<PRE>
$foo = ('cc', '-E', $bar);</PRE>
<P>assigns the value of variable $bar to the scalar variable $foo.
Note that the value of an actual array in scalar context is the
length of the array; the following assigns the value 3 to $foo:</P>
<PRE>
@foo = ('cc', '-E', $bar);
$foo = @foo; # $foo gets 3</PRE>
<P>You may have an optional comma before the closing parenthesis of a
list literal, so that you can say:</P>
<PRE>
@foo = (
1,
2,
3,
);</PRE>
<P>To use a here-document to assign an array, one line per element,
you might use an approach like this:</P>
<PRE>
@sauces = <<End_Lines =~ m/(\S.*\S)/g;
normal tomato
spicy tomato
green chile
pesto
white wine
End_Lines</PRE>
<P>LISTs do automatic interpolation of sublists. That is, when a LIST is
evaluated, each element of the list is evaluated in list context, and
the resulting list value is interpolated into LIST just as if each
individual element were a member of LIST. Thus arrays and hashes lose their
identity in a LIST--the list</P>
<PRE>
(@foo,@bar,&SomeSub,%glarch)</PRE>
<P>contains all the elements of @foo followed by all the elements of @bar,
followed by all the elements returned by the subroutine named SomeSub
called in list context, followed by the key/value pairs of %glarch.
To make a list reference that does <EM>NOT</EM> interpolate, see <A HREF="../../lib/Pod/perlref.html">the perlref manpage</A>.</P>
<P>The null list is represented by (). Interpolating it in a list
has no effect. Thus ((),(),()) is equivalent to (). Similarly,
interpolating an array with no elements is the same as if no
array had been interpolated at that point.</P>
<P>A list value may also be subscripted like a normal array. You must
put the list in parentheses to avoid ambiguity. For example:</P>
<P>A slice of an empty list is still an empty list. Thus:</P>
<PRE>
@a = ()[1,0]; # @a has no elements
@b = (@a)[0,1]; # @b has no elements
@c = (0,1)[2,3]; # @c has no elements</PRE>
<P>But:</P>
<PRE>
@a = (1)[1,0]; # @a has two elements
@b = (1,undef)[1,0,2]; # @b has three elements</PRE>
<P>This makes it easy to write loops that terminate when a null list
is returned:</P>
<PRE>
while ( ($home, $user) = (getpwent)[7,0]) {
printf "%-8s %s\n", $user, $home;
}</PRE>
<P>As noted earlier in this document, the scalar sense of list assignment
is the number of elements on the right-hand side of the assignment.
The null list contains no elements, so when the password file is
exhausted, the result is 0, not 2.</P>
<P>If you're confused about why you use an '@' there on a hash slice
instead of a '%', think of it like this. The type of bracket (square
or curly) governs whether it's an array or a hash being looked at.
On the other hand, the leading symbol ('$' or '@') on the array or
hash indicates whether you are getting back a singular value (a
scalar) or a plural one (a list).</P>
<P>
<H2><A NAME="typeglobs and filehandles">Typeglobs and Filehandles</A></H2>
<P>Perl uses an internal type called a <EM>typeglob</EM> to hold an entire
symbol table entry. The type prefix of a typeglob is a <CODE>*</CODE>, because
it represents all types. This used to be the preferred way to
pass arrays and hashes by reference into a function, but now that
we have real references, this is seldom needed.</P>
<P>The main use of typeglobs in modern Perl is create symbol table aliases.
This assignment:</P>
<PRE>
*this = *that;</PRE>
<P>makes $this an alias for $that, @this an alias for @that, %this an alias
for %that, &this an alias for &that, etc. Much safer is to use a reference.
This:</P>
<PRE>
local *Here::blue = \$There::green;</PRE>
<P>temporarily makes $Here::blue an alias for $There::green, but doesn't
make @Here::blue an alias for @There::green, or %Here::blue an alias for
%There::green, etc. See <A HREF="../../lib/Pod/perlmod.html#symbol tables">Symbol Tables in the perlmod manpage</A> for more examples
of this. Strange though this may seem, this is the basis for the whole
module import/export system.</P>
<P>Another use for typeglobs is to pass filehandles into a function or
to create new filehandles. If you need to use a typeglob to save away
a filehandle, do it this way:</P>
<PRE>
$fh = *STDOUT;</PRE>
<P>or perhaps as a real reference, like this:</P>
<PRE>
$fh = \*STDOUT;</PRE>
<P>See <A HREF="../../lib/Pod/perlsub.html">the perlsub manpage</A> for examples of using these as indirect filehandles
in functions.</P>
<P>Typeglobs are also a way to create a local filehandle using the <A HREF="../../lib/Pod/perlfunc.html#item_local"><CODE>local()</CODE></A>
operator. These last until their block is exited, but may be passed back.
For example:</P>
<PRE>
sub newopen {
my $path = shift;
local *FH; # not my!
open (FH, $path) or return undef;
return *FH;
}
$fh = newopen('/etc/passwd');</PRE>
<P>Now that we have the <CODE>*foo{THING}</CODE> notation, typeglobs aren't used as much
for filehandle manipulations, although they're still needed to pass brand
new file and directory handles into or out of functions. That's because
<CODE>*HANDLE{IO}</CODE> only works if HANDLE has already been used as a handle.
In other words, <CODE>*FH</CODE> must be used to create new symbol table entries;
<CODE>*foo{THING}</CODE> cannot. When in doubt, use <CODE>*FH</CODE>.</P>
<P>All functions that are capable of creating filehandles (open(),
opendir(), pipe(), socketpair(), sysopen(), socket(), and <A HREF="../../lib/Pod/perlfunc.html#item_accept"><CODE>accept())</CODE></A>
automatically create an anonymous filehandle if the handle passed to
them is an uninitialized scalar variable. This allows the constructs
such as <A HREF="../../lib/Pod/perlfunc.html#item_open"><CODE>open(my $fh, ...)</CODE></A> and <A HREF="../../lib/Pod/perlfunc.html#item_open"><CODE>open(local $fh,...)</CODE></A> to be used to
create filehandles that will conveniently be closed automatically when
the scope ends, provided there are no other references to them. This
largely eliminates the need for typeglobs when opening filehandles
that must be passed around, as in the following example:</P>
<PRE>
sub myopen {
open my $fh, "@_"
or die "Can't open '@_': $!";
return $fh;
}</PRE>
<PRE>
{
my $f = myopen("</etc/motd");
print <$f>;
# $f implicitly closed here
}</PRE>
<P>Another way to create anonymous filehandles is with the Symbol
module or with the IO::Handle module and its ilk. These modules
have the advantage of not hiding different types of the same name
during the local(). See the bottom of <A HREF="../../lib/Pod/perlfunc.html#open()">open() in the perlfunc manpage</A> for an
example.</P>
<P>
<HR>
<H1><A NAME="see also">SEE ALSO</A></H1>
<P>See <A HREF="../../lib/Pod/perlvar.html">the perlvar manpage</A> for a description of Perl's built-in variables and
a discussion of legal variable names. See <A HREF="../../lib/Pod/perlref.html">the perlref manpage</A>, <A HREF="../../lib/Pod/perlsub.html">the perlsub manpage</A>,
and <A HREF="../../lib/Pod/perlmod.html#symbol tables">Symbol Tables in the perlmod manpage</A> for more discussion on typeglobs and
