I'm sure it's possible to pose a challenge ("real project") in which those variables would be needed... but it's hard to come up with one. Consider: 
 $` The text before matching -- In your code, that's $in
 $' The text which comes after the match in an input string -- Alt: Lo
+okarounds
 $& The text of the match itself -- Use captures instead

[download]

As mentioned, the cost (= slowdown) is well documented in the standard regex docs; and in many books, tutorials and nodes devoted to regular expressions.

Concerning your code: given that sub uncomment_one is never explicitly called in what you posted, you may have over-reached in your diligence to follow the guidance ( not exactly "rules ) of this forum. /me suspects that profiling what you show would be informative; certainly, as is, the slowdown using your code is largely caused by the copying which is expensive, as JavaFan points out above.

In reply to Re^3: Impact of special variables on regex match performance by ww
in thread Impact of special variables on regex match performance by roubi

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