________ $1 unconditionally refers to this capture
  /         if the match was successful.
 |
 |       __ $2 unconditionally refers to this capture
 |      /   if the match was successful.
 |     |
 v     v
/(...)|(...)/

##</code><code>##

for (qw(a b)) {
   /(a)|(b)/;
   print(
      defined($1) ? $1 : '~',
      defined($2) ? $2 : '~',
      "\n"
   );
}

##</code><code>##

a~
~b