use strict;

my %rev_hash = ();
my %hash = (
  'key1' => [1, 20, 3],
  'key2' => [1, 30, 4, 7],
  'key3' => [1, 9, 8],
  'key4' => [1, 9, 17],
  'key5' => [1, 9, 17],
);

{
  local ($;) = " ";

  while (my ($k, $v) = each %hash) {
    push @{$rev_hash{"@{$v}"}}, $k;
  }
  
  while (my ($k, $v) = each %rev_hash) {
    print qq(\$rev_hash{$k} = "@{$v}"\n);
  }
}