$ perl -Mre=debug -le " q/aa/ =~ /a\b/ "
Compiling REx "a\b"
Final program:
   1: EXACT <a> (3)
   3: BOUND (4)
   4: END (0)
anchored "a" at 0 (checking anchored) minlen 1
Guessing start of match in sv for REx "a\b" against "aa"
Found anchored substr "a" at offset 0...
Guessed: match at offset 0
Matching REx "a\b" against "aa"
   0 <> <aa>                 |  1:EXACT <a>(3)
   1 <a> <a>                 |  3:BOUND(4)
                                  failed...
   1 <a> <a>                 |  1:EXACT <a>(3)
   2 <aa> <>                 |  3:BOUND(4)
   2 <aa> <>                 |  4:END(0)
Match successful!
Freeing REx: "a\b"