Very good point. This (not very optimised) code would flip and create the data structure. It is a bit cheesy though: 
use Data::Dumper;
my %hash = ( 'a'=>1, 'b'=>2, 'c'=>3, 'd'=>4, 'e'=>4, 'f'=>4);
my %by_value = flip(%hash);
print Data::Dumper->Dump([\%hash], ['*hash']);
print Data::Dumper->Dump([\%by_value], ['*by_value']);

sub flip
{
    my($k, $v, %hash);
    my %foo  = @_;
    $hash{$v} = defined $hash{$v} ? 
                    [ ref $hash{$v} ? @{$hash{$v}} : $hash{$v} , $k ] 
+: 
                    $k while (($k,$v) = each %foo); 
    return %hash;
}

[download]
Results:
%hash = (
          'a' => 1,
          'b' => 2,
          'c' => 3,
          'd' => 4,
          'e' => 4,
          'f' => 4
        );
%by_value = (
              1 => 'a',
              2 => 'b',
              3 => 'c',
              4 => [
                     'd',
                     'e',
                     'f'
                   ]
            );

[download]

In reply to RE: RE: auto-magical flipped hashes by perlmonkey
in thread auto-magical flipped hashes by gregorovius

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