my $tag = 'foo';       # option variable with default value
GetOptions ('tag=s' => \$tag);

##</code><code>##

my $listref = ['a','b','c'];       # option variable with default values
GetOptions ('list=s{,}' => $listref);

##</code><code>##

my $listref = [];       # no default
GetOptions ('list=s{,}' => $listref);
my $listref = @{$listref} ? $listref : ['a', 'b', 'c']; # DIY default