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my %hash          = ( a => 1, b => 2, c => 3 );
my %inverted_hash = reverse %hash;

use Data::Dumper;
print Dumper(\(%hash, %inverted_hash));
...
#           '3' => 'c',
#           '2' => 'b'
#         };

%hash          = ( a => 1, b => 2, c => 2 );
%inverted_hash = reverse %hash;
...
#           '1' => 'a',
#           '2' => 'c'
#         };

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