reasonablekeith,
Here is a clue - what happens when you forget about closures and run the following code?

#!/usr/bin/perl
use strict;
use warnings;

my $cnt = 0;
print $cnt++, "\n";
print $cnt++, "\n";
print $cnt += 10, "\n";
print $cnt++, "\n";
print $cnt++, "\n";
[download]

The ++ operator (see perlop) first retrieves the current value and then increments the value when used as post-increment.

• Start at 0
• Return 0, make $cnt 1
• Return 1, make $cnt 2
• Return 2 + 10, make $cnt 12
• Return 12, make $cnt 13
• Return 13, make $cnt 14