sub format_number {
    die 'Usage:format_number(<number>, <format>)!' unless scalar(@_) == 2;
    
         my ($num, $format)  = @_;
    
         my @form = split('', $format);
         @form = reverse @form;
         my @num  = split('', $num);
         my ($symbol, @res);
         foreach(@form) {
             if(/^([Xx]|#)$/) {
                 unless($symbol = pop @num) {
                     $symbol = 0;
                 }
             } else {
                 $symbol = $_;
             }
    
             unshift @res, $symbol
         }
    
         return join('', @res);
     }