my $count;

'ac' =~
   /
      a (?{ $count++ }) b
      |
      a (?{ $count++ }) c
   /x;

# 1. Matches 'a' in first branch.
# 2. Increments $count to 1.
# 3. Fails to match 'b'.
# 4. Matches 'a' in second branch.
# 5. Increments $count to 2.
# 6. Matches 'c'.

print("$count\n");  # 2