$ perl -MData::Dump -Mre=debug -le " dd( $_=qq{a\n\nb} ); s{^$}{boop}m; dd( $_ ); "
Compiling REx "^$"
Final program:
   1: MBOL (2)
   2: MEOL (3)
   3: END (0)
anchored ""$ at 0 anchored(MBOL) minlen 0
"a\n\nb"

Matching REx "^$" against "a%n%nb"
   0 <> <a%n%nb>             |  1:MBOL(2)
   0 <> <a%n%nb>             |  2:MEOL(3)
                                  failed...
   2 <a%n> <%nb>             |  1:MBOL(2)
   2 <a%n> <%nb>             |  2:MEOL(3)
   2 <a%n> <%nb>             |  3:END(0)
Match successful!
"a\nboop\nb"

Freeing REx: "^$"