IIRC, once perl sees $& anywhere in the program code, it starts to populate that variable (and $' and $`) for all the regular expression matches in the program. Using it impacts the performance of all the regular expressions in the code, not just those ones where it is actually needed!

In reply to Re^2: substrings that consist of repeating characters by salva
in thread substrings that consist of repeating characters by Anonymous Monk

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