The regex definition  qr{ab[crdnfqm\+\s]def}; does add a  '+' character and the class of all whitespace characters to the  [crdnfqm] character class contained in the previous  qr{ab[crdnfqm]def}; definition. Note, however, that  m{ ($pattern) } (still) requires a blank (0x20) character before and after  $pattern in order to have a match. (NB: The  '+' character is not special, i.e., is not a metacharacter, inside a character class and so does not need the  \ escape — but it does no harm.)

Please see perlre, perlrecharclass, perlrequick, and perlretut.