I found all solutions so far overly complicated, what am I missing?

please note that the maximal length of the queue is only the sum of children at the current descend path, not all tree elements. (e.g. a binary tree with 10 levels has 2**10 = 2014 nodes but the queue will never exceed 20 = 10*2 elements)

Changing from unshift to push will perform a breadth first search.

use strict;
use warnings;
use Data::Dump qw/pp dd/;


print "\n---------- Input\n";

sub genNested {
    my $n = shift;
    map{ rand() < 0.5 ? [ int rand(20), genNested( $n-1 ) ] : int rand
+( 20 ) } 1 ..$n;
}

my @nested = genNested( 3 );

print pp \@nested;



print "\n\n---------- my solution\n";

sub genIterator {
    my $tree = shift;
    my @queue = @{$tree};
    return sub {
        while (  my ($node) = splice @queue,0,1 ) { # shift is buggy
            if ( ref $node eq 'ARRAY' ) {
                unshift @queue, @{$node};
                next;
            } else {
                return $node
            }
        }
        return;
    };
    
}

my $iter = genIterator( \@nested );


while ( my ($next) = $iter->() ) {
    print "$next,\t";
}



print "\n\n---------- BUKs goal\n";

sub cbIterator (&$) {
    my( $code, $tree ) = @_;
    for my $i ( 0 .. $#$tree ) {
        if( ref( $tree->[ $i ] ) eq 'ARRAY' ) {
            &cbIterator( $code, $tree->[ $i ] );
        }
        else {
            local $_ = $tree->[ $i ];
            $code->();
        }
    }
}





cbIterator {
     print "$_,\t";
} \@nested;
[download]

---------- Input
[
  [934, [993, 105], [167, [213]]],
  [71, [120, [973]], 135],
  [404, [828, [571]], 945],
]

---------- my solution
934,    993,    105,    167,    213,    71,    120,    973,    135,   
+ 404,    828,    571,    945,    

---------- BUKs goal
934,    993,    105,    167,    213,    71,    120,    973,    135,   
+ 404,    828,    571,    945,
[download]

update

fixed issue with strange shift behaviour.

update

changed from shift to splice to handle bug with false nodes