% perl
use warnings;
use strict;
use Data::Dumper;
sub f {
    my @tmp = "list $_[0]\n" =~ /^list\s+(\w+)(?:,(\w+))*$/m;
    print(Dumper(\@tmp));
}
f("foo");
f("foo,bar");

# Ctrl-D
$VAR1 = [
          'foo',
          undef
        ];
$VAR1 = [
          'foo',
          'bar'
        ];