An alternate approach is to "pipeline" the process. Split the stream into tokens, operate on the tokens, and then reassemble the tokens. This might be overkill for your particular example, but is still a useful technique to have in your bag. 
  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|

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It's tempting to use 
  split /\|/ $string

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to do the tokenizing, then 
  join "|"

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to reassemble them, but you'll lose the trailing "|".

In reply to Re: Replacing consecutive tokens in 1 pass by dws
in thread Replacing consecutive tokens in 1 pass by aarestad

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