#!/usr/bin/perl -w

use strict;

my $cnt;
my $res;

my $regex = qr/
	       (?{ $cnt = 0 })        # Initialize $cnt.
	       (?:                    # group only
	       a
	       (?{
	       local $cnt = $cnt + 1; # Update $cnt, backtracking-safe.
	       })
	       )*
	       aaaa
	       (?{ $res = $cnt })     # success-copy to non-local var
	       /x;

foreach my $line (<DATA>) {
    chomp $line;

    print STDERR "huzzah |$`| |$&| |$'| $res $line\n" if  $line =~ $regex;
}

__DATA__
aaaaaaaa
aaaa