What is happening in that one liner:

print "$vm, $filer, \n" if ! $seen{"$vm:$filler"}++
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It's quite an elegant piece of code, with a lot going on. So let's break it down:

• The hash called '%seen' is being checked - to see if it contains a key called "$vm:$filler". The value of this is being tested for being 'false'. (because there's 'if !' which is 'if not') - which is literally 'if not seen'
• So if $seen{"$vm:filler"} is _not_ 'true' - we haven't seen it, and so we print it.
• And then that ++ kicks in, to post increment, 'adding one' to $seen{"$vm:$filler"}' - so the first time we try doing that, it's going to be undefined (and thus false) triggering a print. Second time, it'll be 'non zero' and so evaluate as 'true'.

For bonus points, you can probably do:

foreach my $key ( keys %seen )
{
  print "$key was seen $seen{$key} times\n";
}
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Does that make it any clearer?