I share the concerns of others about mixing too much Perl majick together: the result may blow up in your (or your maintainer's) face in very unexpected ways. But as to some tangential questions...

Is there an efficient way to check whether the values of a hash are unique?

c:\@Work\Perl\monks>perl -wMstrict -le
"my %h = qw(a 1  b 2  c 3  d 1);
 ;;
 my %r = reverse %h;
 keys %r == keys %h or die qq{hash @{[ %h ]} not unique};
"
hash c 3 a 1 b 2 d 1 not unique at -e line 1.
[download]

... a solution to return several values if the hash is not unique ...

The general approach to maintaining several values for a hash key is an anonymous array. Maybe something like:

c:\@Work\Perl\monks>perl -wMstrict -le
"use Data::Dump qw(dd);
 ;;
 my %h = (
   a => [ 1 ],  b => [ 2, 99, 88 ],  c => [ 3 ],  d => [ 1, 99 ]
   );
 ;;
 my %r;
 for my $hk (keys %h) {
   push @{ $r{$_} }, $hk for @{ $h{$hk} };
   }
 dd \%r;
"
{ 1 => ["a", "d"], 2 => ["b"], 3 => ["c"], 88 => ["b"], 99 => ["b", "d
+"] }
[download]

Update: Note that this inversion approach for multi-value hashes is not round-tripable unless you consider the arrays of values to be unordered lists:

c:\@Work\Perl\monks>perl -wMstrict -le
"use Test::More 'no_plan';
 use Test::NoWarnings;
 ;;
 use Data::Dump qw(dd);
 ;;
 my %h = (
   a => [ 1 ], b => [ 2, 99, 88 ], c => [ 3 ], d => [ 1, 99 ]
   );
 ;;
 my %r  = invert(%h);
 dd \%r;
 ;;
 my %rr = invert(%r);
 dd \%rr;
 ;;
 is_deeply \%rr, \%h, 'round trip';
 ;;
 done_testing;
 ;;
 exit;
 ;;
 sub invert {
   my %h = @_;
   my %r;
   for my $hk (keys %h) {
     push @{ $r{$_} }, $hk for @{ $h{$hk} };
     }
   return %r;
   }
"
{ 1 => ["a", "d"], 2 => ["b"], 3 => ["c"], 88 => ["b"], 99 => ["b", "d
+"] }

{ a => [1], b => [99, 88, 2], c => [3], d => [99, 1] }

not ok 1 - round trip
#   Failed test 'round trip'
#   at -e line 1.
#     Structures begin differing at:
#          $got->{b}[0] = '99'
#     $expected->{b}[0] = '2'
1..1
ok 2 - no warnings
1..2
# Looks like you failed 1 test of 2.
[download]

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