my $given = $ARGV[0];
# ...
while (<$dict_fh>) {
    chomp;
    print "$_\n" if is_subset($_, $given);
}
sub is_subset {
    my ($str1, $str2) = @_;
    my (%h1, %h2);
    $h1{$_}++ for split //, $str1;
    $h2{$_}++ for split //, $str2;
    for my $chr (keys %h2) {
        return if ! $h1{$chr} || $h2{$chr} > $h1{$chr};
    }
    return 1;
}