sub foo
   {
      # ....initialization stuff...., blah blah

      while( my $line = <SOURCE> ) {
         if( $line =~ /<!--a -->/ ) { # could do index(), if looking for exact match like this
            sub_a();
            last;
         } elsif( $line =~ /<!--b -->/ ) { # ditto
            sub_b();
            last;
         }
      }
   }