$do_the_change_mon = sub { 
   $thingy = shift;
   $thingy =~ tr|/|_|;
   return $thingy;
}

$expr = s|
\[([A-Z/]+)\s([A-Z/]+)\s([A-Z/]+)\s([A-Z/]+)\s\]
|&$do_the_change_mon("$1_$2_$3_$4)|xe;