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

Compiling REx "^%nn?"
Final program:
   1: MBOL (2)
   2: EXACT <\n> (4)
   4: CURLY {0,1} (8)
   6:   EXACT <n> (0)
   8: END (0)
anchored "%n" at 0 (checking anchored) anchored(MBOL) minlen 1
Guessing start of match in sv for REx "^%nn?" against "a%n%nb"
Found anchored substr "%n" at offset 1...
Found /^/m, restarting lookup for check-string at offset 2...
Found anchored substr "%n" at offset 2...
Position at offset 2 does not contradict /^/m...
Guessed: match at offset 2
Matching REx "^%nn?" against "%nb"
   2 <a%n> <%nb>             |  1:MBOL(2)
   2 <a%n> <%nb>             |  2:EXACT <\n>(4)
   3 <a%n%n> <b>             |  4:CURLY {0,1}(8)
                                  EXACT <n> can match 0 times out of 1...
   3 <a%n%n> <b>             |  8:  END(0)
Match successful!
"a\nboopb"

Freeing REx: "^%nn?"