Do you know what kinds of things can go wrong?

Backtracking can screw things up:

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
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The fix is to use local. When the regexp backtracks through a local, the old value is restored. The old value is also restored when the regexp succesfully matches, so you need to save the result.

my $count;
our $c = 0;

'ac' =~
   /
      (?:
         a (?{ local $c = $c + 1 }) b
         |
         a (?{ local $c = $c + 1 }) c
      )
      (?{ $count = $c })  # Save result.
   /x;

# 1. Matches 'a' in first branch.
# 2. Increments $c to 1.
# 3. Fails to match 'b'.
# 4. Undoes increment ($c = 0).
# 5. Matches 'a' in second branch.
# 6. Increments $c to 1.
# 7. Matches 'c'.
# 8. $count = $c.

print("$count\n");  # 1
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