my $string = "|90|93|foo|bar|91|92|95|96|906|";

  my $result = join "",
               map { s/^9[0-6]$/X/; $_ }
               $string =~ m/(\||[^|]+)/g;

  print $result;
  __END__
  |X|X|foo|bar|X|X|X|X|906|

##</code><code>##

  split /\|/ $string

##</code><code>##

  join "|"