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$ perl -Mre=debug -e " $f=q{a??q}; $f =~ m{\Qa??\E}; "
Compiling REx "a\?\?"
Final program:
...
Guessed: match at offset 0
Freeing REx: "a\?\?"

$ perl -le " print qr{\Qa??\E}; "
(?^:a\?\?)

$ perl -le " print qr'\Qa??\E'; "
(?^:\Qa??\E)

$ perl -Mre=debug -e " $f=q{a??q}; $f =~ m'\Qa??\E'; "
Compiling REx "\Qa??\E"
...
Did not find floating substr "E"...
Match rejected by optimizer
Freeing REx: "\Qa??\E"