That won't work. What he needs is a positive look-ahead assertion, like this:

use strict;
$_ = "|90|93|foo|bar|91|92|95|96|906|";
s{
    \|9
    [0-6]+
    (?=\|)
}
{|X}xg;
print $_,"\n";
[download]

You also can't use 'x' mode to ignore whitespace in the replacement part. I also fixed that.