Pug,
What you are using is a one-use closure. Because it is in a naked block it can not be used multiple times. And as broquaint points out, it must be "seen" prior to being used. Here is an example of a re-usable closure that can be used before or after the sub has been "seen".

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

my $count_1  = iterator( 1 );
print $count_1->(), "\n" for 1 .. 10;

my $count_2  = iterator( 'a' );
print $count_2->(), "\n" for  1 .. 10;

sub iterator {
    my $iterator = shift;
    return sub { $iterator++ };
}
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Cheers - L~R