sub replace_keep_whitespace {
    my ($str, $word, $rep) = @_;

    $str =~ join('\\n*', split //, $word);

    my $match = \substr($str, $-[0], $+[0] - $-[0]);

    my $c = 0;
    $$match =~ s/[^\n]/substr($rep, $c++, 1)/eg;

    return $str;
  }


  my $str = <<'_DATA_';
  s
  kkjn




  find
  ue
  k


  asd
  helkjadfoijejdklk
  _DATA_

  my $word = 'uekasdh';
  my $rep = 'j' x length $word;

  print replace_keep_whitespace($str, $word, $rep);

  __END__
  s
  kkjn




  find
  jj
  j


  jjj
  jekjadfoijejdklk