## this is a poorly written demonstrative example

use GetOpt::Long;

my $args = {
        verbose         => 0,   # default false
        all             => '',  # default undef
        skip            => [],  # default empty
};

GetOptions(
        'verbose|v'     => sub { \$args->{verbose} += 1 },
        'debug|d=i'     => \$args->{verbose},
        'all'           => \$args->{all},
        'skip|s=s'      => sub { push(@{$args->{skip}}, $_[1]) }
);

# get a default from the environment.
$args->{all} = $ENV{FOO_DEFAULT_ALL} unless (defined($args->{all}));

# get a default from a parsed configuration file.
$args->{all} = $rcfile->{foo_default}->{all} unless (defined($args->{all}));

# get defaults from some arbitrary source.
get_foo_defaults($args);