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.