use strict;
 use Data::Dumper;

 my $SIZE  = shift || 4;
 my @BOARD = map {[(1) x $SIZE]} (1..$SIZE);
 my @SOLUTION;

 # i know that the second argument seems unecessary,
 # but this will used to allow different threads to
 # tackle different starting rows ...
 scan(0,0,[@BOARD],[]);

 print Dumper \@SOLUTION;

 sub scan {
    my ($col,$start_row,$board,$possible) = @_;
    my $copy = [map[@$_],@$board];

    # no more columns? 
    if ($col == $SIZE) {
       # found our solution!
       push @SOLUTION,[@$possible];
       return;
    }

    # find first available row
    for my $row ($start_row..$SIZE-1) {
       if($board->[$row]->[$col]) {

          push @$possible,$row;
          print "available row: $row in col $col: (@$possible)\n";

          mark_attacks($row,$col,$board);
          print_matrix($board);

          scan($col+1,0,[@$board],$possible);

          # i thought that this copy should not even be necessary
          @{$board} = map[@$_],@$copy;

          pop @$possible;
       }
    }

    return;
 }

 sub mark_attacks {
    my ($r,$c,$array) = @_;

    $array->[$r]->[$c] = 'Q';

    # mark horizontal
    $array->[$r]->[$_] = 0 for ($c+1..$SIZE-1);
    # this line will produce 1 solution for n=4 or 5
    #$array->[$r] = [ map {0} @{$array->[$r]} ];

    # mark r-c diagonal
    $array->[--$r]->[++$c] = 0 while ($r > 0) && ($c < $SIZE-1);

    # mark r+c diagonal
    ($r,$c) = @_;
    $array->[++$r]->[++$c] = 0 while ($r < $SIZE-1) && ($c < $SIZE-1);
 }

 sub print_matrix { print join('',@$_),"\n" for @{+shift} }
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