$ perl -Mre=debug -e " $f=q{a??q}; $f =~ m'\Qa??\E'; "
Compiling REx "\Qa??\E"
Final program:
   1: EXACT <Q> (3)
   3: MINMOD (4)
   4: CURLY {0,1} (8)
   6:   EXACT <a> (0)
   8: EXACT <E> (10)
  10: END (0)
anchored "Q" at 0 floating "E" at 1..2 (checking floating) minlen 2
Guessing start of match in sv for REx "\Qa??\E" against "a??q"
Did not find floating substr "E"...
Match rejected by optimizer
Freeing REx: "\Qa??\E"