$str =~ s/.*nr(\d+)([^\d]+)(\d+)(.*)/"calc(nr$1)$2".eval{$3*4}."$4";/e;

FWIW: The separate  eval{$3*4} is not needed (although it does no harm) because the entire replacement expression is already being eval-ed:
    $str =~ s/.*nr(\d+)([^\d]+)(\d+)(.*)/"calc(nr$1);$2" . $3*4 . $4/e;
(tested, and I also stuck the semicolon back where kepler seems to want it).