use strict;
use warnings;
use 5.010;

sub gen_iter {
    my $a = shift;
    sub { $a++ };
}

my $it1 = gen_iter 10;
my $it2 = gen_iter 20;
say $it1->();
say $it2->();
say $it1->();
say $it2->();
__END__
10
20
11
21