shorter example

interpolation, the \Q\E is interpolated in double quoted strings

$ 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\?\?"
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use qr to see how \Q\E gets interpolated

$ perl -le " print qr{\Qa??\E}; "
(?^:a\?\?)
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without interpolation the pattern is completely different

$ perl -le " print qr'\Qa??\E'; "
(?^:\Qa??\E)
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The regex metacharcters are still regex metacharacters (pattern)

$ 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"
[download]

perlrebackslash :)