$ perl -MData::Dump -Mre=debug -le " dd( $_=qq{a\n\nb} ); s{^$\n}{boop}m; dd( $_ ); "
"a\n\nb"

Compiling REx "^%nn"
Final program:
   1: MBOL (2)
   2: EXACT <\nn> (4)
   4: END (0)
anchored "%nn" at 0 (checking anchored) anchored(MBOL) minlen 2
Guessing start of match in sv for REx "^%nn" against "a%n%nb"
Did not find anchored substr "%nn"...
Match rejected by optimizer
"a\n\nb"

Freeing REx: "^%nn"