#!/usr/bin/perl -wl
sub gen_counter {
    my $a = shift;
    return sub { ++$a }, sub { $a = shift };
}

my ($counter, $modifier) = gen_counter(3);
print $counter->();
print $counter->();

$modifier->(8);

print $counter->();
print $counter->();

# which prints:
4
5
9
10