=head1 DESCRIPTION
=head2 Variable names
+X<variable, name> X<variable name> X<data type> X<type>
Perl has three built-in data types: scalars, arrays of scalars, and
associative arrays of scalars, known as "hashes". A scalar is a
for a simple identifier, an expression that produces a reference
to the value at runtime. This is described in more detail below
and in L<perlref>.
+X<identifier>
Perl also has its own built-in variables whose names don't follow
these rules. They have strange names so they don't accidentally
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 L<perlvar>.
+X<variable, built-in>
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.
+X<scalar>
$days # the simple scalar value "days"
$days[28] # the 29th element of array @days
Entire arrays (and slices of arrays and hashes) are denoted by '@',
which works much like the word "these" or "those" does in English,
in that it indicates multiple values are expected.
+X<array>
@days # ($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'})
Entire hashes are denoted by '%':
+X<hash>
%days # (key1, val1, key2, val2 ...)
and @foo are two different variables. It also means that C<$foo[1]>
is a part of @foo, not a part of $foo. This may seem a bit weird,
but that's okay, because it is weird.
+X<namespace>
Because variable references always start with '$', '@', or '%', the
"reserved" words aren't in fact reserved with respect to variable
from conflict with future reserved words. Case I<is> significant--"FOO",
"Foo", and "foo" are all different names. Names that start with a
letter or underscore may also contain digits and underscores.
+X<identifier, case sensitivity>
+X<case>
It is possible to replace such an alphanumeric name with an expression
that returns a reference to the appropriate type. For a description
id.)
=head2 Context
+X<context> X<scalar context> X<list context>
The interpretation of operations and values in Perl sometimes depends
on the requirements of the context around the operation or value.
context.
=head2 Scalar values
+X<scalar> X<number> X<string> X<reference>
All data in Perl is a scalar, an array of scalars, or a hash of
scalars. A scalar may contain one single value in any of three
the null string or the number 0 (or its string equivalent, "0"). The
Boolean context is just a special kind of scalar context where no
conversion to a string or a number is ever performed.
+X<boolean> X<bool> X<true> X<false> X<truth>
There are actually two varieties of null strings (sometimes referred
to as "empty" strings), a defined one and an undefined one. The
use the defined() operator to determine whether a scalar value is
defined (this has no meaning on arrays or hashes), and the undef()
operator to produce an undefined value.
+X<defined> X<undefined> X<undef> X<null> X<string, null>
To find out whether a given string is a valid non-zero number, it's
sometimes enough to test it against both numeric 0 and also lexical
an array that was previously shortened does not recover values
that were in those elements. (It used to do so in Perl 4, but we
had to break this to make sure destructors were called when expected.)
+X<$#> X<array, length>
You can also gain some minuscule measure of efficiency by pre-extending
an array that is going to get big. You can also extend an array
the last value, like the C comma operator, nor of built-in functions,
which return whatever they feel like returning.) The following is
always true:
+X<array, length>
scalar(@whatever) == $#whatever - $[ + 1;
the value of C<$[> no longer need to worry about whether another
file changed its value. (In other words, use of C<$[> is deprecated.)
So in general you can assume that
+X<$[>
scalar(@whatever) == $#whatever + 1;
10,000 of your items. This isn't supposed to happen. If a tied hash
is evaluated in scalar context, a fatal error will result, since this
bucket usage information is currently not available for tied hashes.
+X<hash, scalar context> X<hash, bucket> X<bucket>
You can preallocate space for a hash by assigning to the keys() function.
This rounds up the allocated buckets to the next power of two:
keys(%users) = 1000; # allocate 1024 buckets
=head2 Scalar value constructors
+X<scalar, literal> X<scalar, constant>
Numeric literals are specified in any of the following floating point or
integer formats:
4_294_967_296 # underscore for legibility
0xff # hex
0xdead_beef # more hex
- 0377 # octal
+ 0377 # octal (only numbers, begins with 0)
0b011011 # binary
You are allowed to use underscores (underbars) in numeric literals
between digits for legibility. You could, for example, group binary
digits by threes (as for a Unix-style mode argument such as 0b110_100_100)
or by fours (to represent nibbles, as in 0b1010_0110) or in other groups.
+X<number, literal>
String literals are usually delimited by either single or double
quotes. They work much like quotes in the standard Unix shells:
C<\\>). The usual C-style backslash rules apply for making
characters such as newline, tab, etc., as well as some more exotic
forms. See L<perlop/"Quote and Quote-like Operators"> for a list.
+X<string, literal>
Hexadecimal, octal, or binary, representations in string literals
(e.g. '0xff') are not automatically converted to their integer
names beginning with $ or @, followed by an optional bracketed
expression as a subscript.) The following code segment prints out "The
price is $Z<>100."
+X<interpolation>
$Price = '$100'; # not interpolated
print "The price is $Price.\n"; # interpolated
There is no double interpolation in Perl, so the C<$100> is left as is.
+By default floating point numbers substituted inside strings use the
+dot (".") as the decimal separator. If C<use locale> is in effect,
+and POSIX::setlocale() has been called, the character used for the
+decimal separator is affected by the LC_NUMERIC locale.
+See L<perllocale> and L<POSIX>.
+
As in some shells, you can enclose the variable name in braces to
disambiguate it from following alphanumerics (and underscores).
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:
+X<interpolation>
$who = "Larry";
print PASSWD "${who}::0:0:Superuser:/:/bin/perl\n";
as is any simple identifier within a hash subscript. Neither need
quoting. Our earlier example, C<$days{'Feb'}> can be written as
C<$days{Feb}> and the quotes will be assumed automatically. But
-anything more complicated in the subscript will be interpreted as
-an expression.
+anything more complicated in the subscript will be interpreted as an
+expression. This means for example that C<$version{2.0}++> is
+equivalent to C<$version{2}++>, not to C<$version{'2.0'}++>.
=head3 Version Strings
+X<version string> X<vstring> X<v-string>
B<Note:> Version Strings (v-strings) have been deprecated. They will
-not be available after Perl 5.8. The marginal benefits of v-strings
-were greatly outweighed by the potential for Surprise and Confusion.
+be removed in some future release after Perl 5.8.1. The marginal
+benefits of v-strings were greatly outweighed by the potential for
+Surprise and Confusion.
A literal of the form C<v1.20.300.4000> is parsed as a string composed
of characters with the specified ordinals. This form, known as
comparison operators, C<cmp>, C<gt>, C<lt> etc. If there are two or
more dots in the literal, the leading C<v> may be omitted.
- print v9786; # prints UTF-8 encoded SMILEY, "\x{263a}"
+ print v9786; # prints SMILEY, "\x{263a}"
print v102.111.111; # prints "foo"
print 102.111.111; # same
Such literals are accepted by both C<require> and C<use> for
-doing a version check. The C<$^V> special variable also contains the
-running Perl interpreter's version in this form. See L<perlvar/$^V>.
-Note that using the v-strings for IPv4 addresses is not portable unless
-you also use the inet_aton()/inet_ntoa() routines of the Socket package.
+doing a version check. Note that using the v-strings for IPv4
+addresses is not portable unless you also use the
+inet_aton()/inet_ntoa() routines of the Socket package.
Note that since Perl 5.8.1 the single-number v-strings (like C<v65>)
are not v-strings before the C<< => >> operator (which is usually used
be v-strings always.
=head3 Special Literals
+X<special literal> X<__END__> X<__DATA__> X<END> X<DATA>
+X<end> X<data> X<^D> X<^Z>
The special literals __FILE__, __LINE__, and __PACKAGE__
represent the current filename, line number, and package name at that
will not be interpolated into strings. If there is no current package
(due to an empty C<package;> directive), __PACKAGE__ is the undefined
value.
+X<__FILE__> X<__LINE__> X<__PACKAGE__> X<line> X<file> X<package>
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.
-Text after __DATA__
but may be read via the filehandle C<PACKNAME::DATA>,
+Text after __DATA__ may be read via the filehandle C<PACKNAME::DATA>,
where C<PACKNAME> 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
C<close DATA> 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
+like __DATA__ in the top level script (but not in files loaded with
C<require> or C<do>) and leaves the remaining contents of the
file accessible via C<main::DATA>.
__DATA__ (or __END__) token has not yet been seen.
=head3 Barewords
+X<bareword>
A word that has no other interpretation in the grammar will
be treated as if it were a quoted string. These are known as
by saying C<no strict 'subs'>.
=head3 Array Joining Delimiter
+X<array, interpolation> X<interpolation, array> X<$">
Arrays and slices are interpolated into double-quoted strings
by joining the elements with the delimiter specified in the C<$">
L<perlop/Quote and Quote-like Operators>.
=head2 List value constructors
+X<list>
List values are denoted by separating individual values by commas
(and enclosing the list in parentheses where precedence requires it):
=head2 Subscripts
-An array is subscripted by specifying a dollary sign (C<$>), then the
+An array is subscripted by specifying a dollar sign (C<$>), then the
name of the array (without the leading C<@>), then the subscript inside
square brackets. For example:
print "Darwin's First Name is ", $scientists{"Darwin"}, "\n";
=head2 Slices
+X<slice> X<array, slice> X<hash, slice>
A common way to access an array or a hash is one scalar element at a
time. You can also subscript a list to get a single element from it.
scalar) or a plural one (a list).
=head2 Typeglobs and Filehandles
+X<typeglob> X<filehandle> X<*>
Perl uses an internal type called a I<typeglob> to hold an entire
symbol table entry. The type prefix of a typeglob is a C<*>, because
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