Thanks for pointing out my totally clueless use of qr// (was going on the advice of someone on IRC when I went down that path, instead of RTFM'ing). What I don't understand is- why does the evaling the entire expression work? Does it force interpolation of $lhs and $rhs before the expression is evaluated? In experiments,

$_ = $var;
eval "s/$lhs/$rhs/";
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works, but:

eval "$var =~ /$lhs/$rhs/;";
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doesn't work- what am I missing?