use strict;
use warnings;
use 5.010;              # defined-or
use Data::Dump 'dd';

sub get_iterator {
    my ( $x_to, $y_to, $x_from, $y_from ) = @_;
    $x_from //= 0;
    $y_from //= 0;
    
# store partial solutions to explore, to follow left or right
# branches, respectively

    my @left_agenda  = [[ $x_from, $y_from ]];
    my @right_agenda = [[ $x_from, $y_from ]];
    
    return sub {

LOOP:   {
            return undef unless @left_agenda || @right_agenda;

            my ( $ref, $right );
            unless ( $ref = pop @left_agenda ) {
                $ref   = pop @right_agenda;
                $right = 1
            }
            
            my @path = @$ref;
            my ( $x, $y ) = @{ $path[ -1 ] };

            $x ++;
            $y ++ if $right;
            
            if ( $x <= $x_to and $y <= $y_to ) {
                push @path, [ $x, $y ];
                
                return \@path if $x == $x_to and $y == $y_to;

                push @left_agenda, \@path;
                push @right_agenda, \@path;
            }
            redo LOOP;
        }
    }
}

my $iter = get_iterator( 5, 2 );
my $solution;
dd $solution while $solution = $iter-> ();