#!/usr/bin/perl
use strict;
use warnings;
use re qw/eval /;

my $pattern = q/(.)(?{
        print ++$counts[0];
})^/;

my $line = 'ab';

for (0..2) {
    my @counts = (0);

    print "$_: ";
    # $pattern .= '(?=.)';
    $line =~ /$pattern/;
    print "; \@counts = (", join(', ', @counts), ")\n";
}
print "\@main::counts = (", join(', ', @::counts), ")\n";