Just a quick addendum to the fine answers given above...

I think it would have been helpful for you to try to eliminate the Tk aspect, at which point you may have spotted that the problem was (as Perl told you) a basic syntax error, and nothing to do with Tk itself. As you knew that the error was in line 17, cutting and pasting the relevant code portion into another file (or simply commenting out the surrounding bits) would have given you...

my @buttons = ( '9','8','7','6','5','4','3','2','1','0');
for (my $i = 9; $i >= 0; $i--) 
{
    my $button{$i} = $buttons[$i]; # for instance
}
[download]

...which would produce the same syntax error, but would be easier to disect. In addition, playing with just this portion of the code may give you ideas on other ways of producing the results you require. (For instance, it's possible to eliminate the @buttons array completely, should you wish...)

Cheers,
Ben.