$ perl -Mre=debug -e " $f=q{a??q}; $f =~ m{\Qa??\E}; "
Compiling REx "a\?\?"
Final program:
   1: EXACT <a??> (3)
   3: END (0)
anchored "a??" at 0 (checking anchored isall) minlen 3
Guessing start of match in sv for REx "a\?\?" against "a??q"
Found anchored substr "a??" at offset 0...
Guessed: match at offset 0
Freeing REx: "a\?\?"