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"] }