Solution 

• Get the algorithm right, then play golf

• Version 1

#!/usr/bin/perl
while (<>) {
    chomp;
    ($a, $b) = split /\./;
    $h{"$a.$b"} = 1;
    $h{"$a.M"} = 1;
}
print join("\n", sort keys %h), "\n";

• Problem: while(<>){...} is just wrong

• Solution: perl -n - assume "while (<>) { ... }" loop around program