use strict; 
use warnings;

# Merge takes two iterators, which can be called
# with no arguments, in which case they iterate, returning
# their next value; or with a string 'peek', which will
# yield the next value without effectively shifting it.
# Exhausted iterators return undef on all subsequent calls
# 
# Merge itself is an iterator, returning the next element in
# the merged series, plus a continuation coderef
sub merge {
    my ($a, $b) = @_;
    my ($car_a, $car_b) = ($a->('peek'), $b->('peek'));
    # Base case: if one of them is empty, return the other
    defined $car_a or return ($b->(), sub {merge($a, $b)});
    defined $car_b or return ($a->(), sub {merge($a, $b)});

    # Pull off lesser (both if equal) first element(s)
    my $low_car;
    if ($car_a <= $car_b) { $low_car = $a->() }
    if ($car_b <= $car_a) { $low_car = $b->() }

    return ($low_car, sub {merge($a, $b)} );
}

my $I1 = do {
    my @arr = (1..10);
    sub {
        if (@arr == 0) {
            return undef;
        }
        elsif (@_) { # should be peek, but any other arg works (undocumented)
            return $arr[0];
        }
        else {
            return shift @arr;
        }
    }
};

my $I2 = do {
    my $i = 2;
    sub {
        if (@_) {
            return $i;
        } else {
            my $i_was = $i; $i += 2; return $i_was;
        }
    }
};

my $iterations_to_print = 30;
for ( my ($elem, $cont) = merge($I1, $I2);
      $iterations_to_print-- > 0;
      ($elem, $cont) = $cont->()
    ) {
    print "<$elem>\n";
}