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;
}