The usage is quite obvious once you start the program. First enter the numbers, then click button "think". This version only displays one solution if multiple ones exist (if you read the code, you will find that it actually finds out all solutions, although only one is displayed). The number you entered are in black, when computer gives its numbers in red.

use Tk;
use Tk::Table;

use strict;
use warnings;

my $mw = MainWindow->new;
my $table = $mw->Table(-rows => 9, -columns => 9, -fixedrows => 9, fix
+edcolumns => 9)->pack;
foreach my $row (0 .. 8) {
    foreach my $col (0 .. 8) {
        if ((int($row/3) + int($col/3)) % 2) {
            $table->put($row, $col, $mw->Entry(-width => 1, -font=>"Ad
+obe 30 bold", -background => "grey"));
        } else {
            $table->put($row, $col, $mw->Entry(-width => 1, -font=>"Ad
+obe 30 bold", -background => "white"));
        }
    }
}
$mw->Button(-text => "Think", -command => \&think)->pack();


MainLoop;


sub think {
    my $board;
    foreach my $row (0 .. 8) {
        foreach my $col (0 .. 8) {
            $board->[$row][$col] = $table->get($row, $col)->get();
            $board->[$row][$col] = undef if (!($board->[$row][$col]));
        }
    }
    try($board);
}

sub try {
    my $board = shift;
    my ($x, $y, @options) = find_blank_with_least_options($board);
    if (defined($x)) {
        for (@options) {
            $board->[$x][$y] = $_;
            try($board);
        }
        $board->[$x][$y] = undef;
    } else {
        print "find solution:\n";
        display($board);
    }
}

sub find_blank_with_least_options {
    my $board = shift;
    my ($x_to_return, $y_to_return, @options_to_return);
    my $least = 9;
    for my $x (0 .. 8) {
        for my $y (0 .. 8) {
            if (!defined($board->[$x][$y])) {
                my @options = (0,1,1,1,1,1,1,1,1,1);
                for (0 .. 8) {
                    $options[$board->[$x][$_]] = 0 if defined($board->
+[$x][$_]);
                    $options[$board->[$_][$y]] = 0 if defined($board->
+[$_][$y]);
                }
                for my $i (int($x/3) * 3 .. int($x/3) * 3 + 2) {
                    for my $j (int($y/3) * 3 .. int($y/3) * 3 + 2) {
                        $options[$board->[$i][$j]] = 0 if defined($boa
+rd->[$i][$j]);
                    }
                }
                my $sum;
                $sum += $options[$_] for (0 .. 9);
                if ($sum < $least) {
                    $x_to_return = $x;
                    $y_to_return = $y;
                    $least = $sum;
                    $#options_to_return = -1;
                    for (1 .. 9) {
                        push @options_to_return, $_ if ($options[$_]);
                    }
                }
            }
        }
    }
    return ($x_to_return, $y_to_return, @options_to_return);
}

sub display {
    my $board = shift;
    for my $row (0 .. 8) {
        for my $col (0 .. 8) {
            if (!$table->get($row, $col)->get()) {
                $table->get($row, $col)->configure(-foreground => "red
+");
                $table->get($row, $col)->insert('@0', $board->[$row][$
+col]);
            }
        }
    }
}
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