If I'm reading correctly (Update: I wasn't reading correctly), you're substituting all occurrences of "!" with a "\n" newline as long as it falls within the portion of the string that comes before the 50,000th position.

substr( $group, 0, 50000 ) =~ tr/!/\n/;
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That's about the best I can come up with. You're using substr as an lvalue so that the change propagates back to $group, but is constrained to just the range specified. And you're using tr/// which is faster than s/// for single-character transliteration, where a search pattern isn't required.

Update: Bah, I can see already that I misread what you're doing. Looks more like you want to replace the first 50k "!" characters with newlines, not all "!" characters that reside in the first 50k positions. Pardon me. ;)

Update 2: Here's a version that will substitute all '!' characters with \n, up to 50k times. After that, it will no longer match. It will be faster, but not necessarily legible:

$group =~ s/!(??{ ( $myregexp::count++ < 50000 ) ? '' : '(?!)' })/\n/g
+;
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