$_="abcdef"; s/@{[s".."."gr]}/$& /; print;
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This variation of yours does in fact work (prints "abc def"), and so do the following ones (which are closer to the original solution by "tails")

Not for me. It prints "...abcdef" (with perl v5.14.4).

Thus I conclude that the original "solution" is the result of a typo (separator '/' put in the wrong place), and cannot work.

No typo. Code is good. You can test it on: http://golf.shinh.org/p.rb?Bisection