$hash = {
  1 => {
    key1 => string1,
    key2 => string2
  },
  2 => {
    key1 => string3,
    key2 => string4
  }
}

##</code><code>##

foreach my $key (sort { $hash->{'key1'} {$a} cmp $hash->{'key1'} {$b} } keys %$hash) {
  ...
}