OK, to your first problem

It [@counts] is first used outside the for-loop

this is inside a single quoted string. It is the same as if I put it directly into the regex in the loop or declare it inside the loop. The reason I'm doing it this way is that I want to change this pattern inside the loop for my third testcase.

Name "main::counts" used only once: possible typo at ./re-code.pl line
+ 20.
0: 12; @counts = (0)
1: 34; @counts = (0)
2: 56; @counts = (0)
@main::counts = (6)
[download]

This is very interesting. As I interpret it, the code block inside the regex uses the global variable @main::counts which is otherwise blocked from view inside the loop by the my @counts declaration. A simple equivalent example

{
  my $num = 0;
  $main::num = 5; # this instead of the regex
  print $num;     # prints 0
}
print $num;       # prints 5

# or under use strict
print $main::num; # prints 5 as well
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Makes perfect sense. However with 5.6.1 you seem to be able to use lexical variables from the enclosing scope, but this is where the bug comes in. It works the first time but doesn't work the next times.

btw, the warning can be ignored in this case

-- Hofmator