On the subject of inverting hashes, you can invert a hash via reverse like so:

my %hash          = ( a => 1, b => 2, c => 3 );
my %inverted_hash = reverse %hash;
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To see the results, we can use Data::Dumper:

use Data::Dumper;
print Dumper(\(%hash, %inverted_hash));

# Output:
# $VAR1 = {
#           'c' => 3,
#           'a' => 1,
#           'b' => 2
#         };
# $VAR2 = {
#           '1' => 'a',
#           '3' => 'c',
#           '2' => 'b'
#         };
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As you note, however, not all hashes have one-to-one mappings to their inverted forms. In such a case the inverted form will have fewer key-value pairs than the original hash:

%hash          = ( a => 1, b => 2, c => 2 );
%inverted_hash = reverse %hash;

print Dumper(\(%hash, %inverted_hash));

# Output:
# $VAR1 = {
#           'c' => 2,
#           'a' => 1,
#           'b' => 2
#         };
# $VAR2 = {
#           '1' => 'a',
#           '2' => 'c'
#         };
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You can use this property to test whether you have lost information during inversion:

print "keys were lost during inversion\n"
    if keys %inverted_hash < keys %hash;
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Cheers,
Tom