use strict;
use warnings;
use Tie::Hash::Regex;

tie my %hash, 'Tie::Hash::Regex';

%hash = (
   'foo.txt'  => 1,
   'bar.html' => 2,
   '..'       => undef,
   '.'        => undef,
   DEFAULT    => undef,
);

my $arg = shift || 'DEFAULT';
my $candidate = $hash{$arg};

die "invalid arg\n" unless $candidate;
print "$candidate\n";