s/(.)(?=.)/$2$1/g;
...
... produced 5 warnings and left the string unchanged (I presume the warnings were that $2 was undef)..

Yes, because there is no capture group 2 ((?=.) is a non-capturing look-ahead) and the first capture group is being replaced with itself: $1.

In reply to Re: rotate a vector with a regex? by AnomalousMonk
in thread rotate a vector with a regex? by misterperl

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