Suppose you have %hash, and you want to show all the keys that share the same given value. %revhash = reverse %hash won't work because of the value collision. Solution: use arrayrefs for values during the inversion!
my %revhash;
while (my ($key, $value) = each %hash) {
  push @{$revhash{$value}}, $key;
}
for (sort keys %revhash) {
  print "$_ => @{$revhash{$_}}\n";
}

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(Revised per mdillon's note, but in a different way for speed)

In reply to Inverting a hash to get all keys for this value by merlyn

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