# 13 is in list 1
# 23 is in list 5 and 6
# 42 is in list 1,3 and 5

my %lists;
@lists{13,23,42}=([1],[5,6],[1,3,4]);

# Find which lists contain the values
for my $i (13,23,42) {
  print "Value $i found in list $_\n" foreach (@{$lists{$i}})
}