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Sets the variables in optionlist to change
the default behavior of GetOptions. The following
options are available:

$Getopt::Long::autoabbrev
If true, option names can be invoked with unique abbreviations.
Default is 1 (true) unless the environment
variable POSIXLY_CORRECT has been set.

$Getopt::Long::getopt_compat
If true, options can start with +. Default is
1 unless the environment variable
POSIXLY_CORRECT has been set.

$Getopt::Long::order
Value indicates whether options and non-options may be mixed on the
command line:

$PERMUTE
Non-options may be mixed with options. The default if
POSIXLY_CORRECT is not set.

$REQUIRE_ORDER
Mixing is not allowed. The default if
POSIXLY_CORRECT is set.

$Getopt::Long::ignorecase
If true, ignore case when matching options. Default is
1.

$Getopt::Long::VERSION
The version number of this Getopt::Long implementation in the format
major.minor.

$Getopt::Long::error
Internal error flag. May be incremented from a callback routine to
cause options-parsing to fail.

$Getopt::Long::debug
If true, enables debugging output. Default is 0
(false).

Uses descriptions from option-descriptionsto retrieve and process the command-line options with which your Perl
program was invoked. The options are taken from
@ARGV. After GetOptions has
processed the options, @ARGV contains only
command-line arguments that were not options. Returns
0 if errors are detected. Each option description
consists of two elements:

Option specifier
Defines the option name and optionally a value as an argument
specifier.

Option linkage
A reference to a variable that is set when the option is present.

GetOptions can also take as a first argument a
reference to a hash that describes the linkage for the options. The
linkage specified in the argument list takes precedence over the one
specified in the hash. Thus, the following are equivalent:

%optctl = (size => \$offset);
&GetOptions(\%optctl, "size=i");

and:

&GetOptions("size=i" => \$offset);

Option specifiers

Each option specifier consists of an option name and possibly an
argument specifier. The name can be a name, or a list of names
separated by |; the first name in the list is the
true name of the option, and the others are treated as aliases.
Option names may be invoked with the shortest unique abbreviation.
Values for argument specifiers are:

<none>
Option takes no argument. The option variable is set to
1.

!
Option does not take an argument and may be negated, that is,
prefixed by no.

=s
Option takes a mandatory argument that is a string that will be
assigned to the option variable. Even if the argument starts with
- or --, it is assigned to the
option variable rather than treated as another option.

:s
Option takes an optional string argument. If the option is invoked
with no argument, an empty string
(") is assigned to the option
variable. If the argument starts with - or
--, it is treated as another option rather than
assigned to the option variable.

=i
Option takes a mandatory integer argument, which may start with
- to indicate a negative value.

:i
Option takes an optional integer argument that may start with
- to indicate a negative value. With no argument,
the value 0 is assigned to the option variable.

=f
Option takes a mandatory floating-point argument that may start with
- to indicate a negative value.

:f
Option takes an optional floating-point argument that may start with
- to indicate a negative value. With no argument,
the value 0 is assigned to the option variable.

A hyphen (-) by itself is considered an option
whose name is the empty string. A double hyphen
(--) by itself terminates option processing. Any
options following the double hyphen remain in
@ARGV when GetOptions returns.
If an argument specifier ends with @ (e.g.,
=s@), then the option is treated as an array.

The special option specifier <> can be used
to designate a subroutine to handle non-option arguments. For this
specifier to be used, the variable
$Getopt::Long::order must have the value of the
predefined and exported variable, $PERMUTE. See
the description of Getopt::Long::config below.

Linkage specification

The linkage specifier can be a reference to any of the following:

Scalar
The new value is stored in the referenced variable. If the option
occurs more than once, the previous value is overwritten.

Array
The new value is appended (pushed) onto the referenced array.

Subroutine
The referenced subroutine is called with two arguments: the option
name, which is always the true name, and the option value.

If no linkage is explicitly specified, but a hash reference is
passed, GetOptions puts the value in the hash. For
array options, a reference to an anonymous array is generated.

If no linkage is explicitly specified, and no hash reference is
passed, GetOptions puts the value into a global
variable named after the option, prefixed by opt_.
Characters that are not part of the variable syntax are translated to
underscores. For example, --fpp-struct-return sets
the variable $opt_fpp_struct_return.

8.116. GDBM_File		8.118. Getopt::Std