use strict;
use warnings;

while (<DATA>) {
    chomp;
    
    next unless length;
    print "$_ is not a number that I can handle\n" unless /^\d+$/;
    print "$_ = ", toWords("00$_"), "\n";
}


sub toWords {
    my ($leftBit, $groups) = @_;
    
    return '' unless $leftBit;
    
    $groups ||= 0;
    
    my $group = substr $leftBit, -3, 3, '';
    
    return toWords ($leftBit, $groups + 1) unless $group;

    if ($groups > 4) {
        print "Argh, I can't deal with numbers that big!\n";
        print "The part I can deal with is: ";
        return '';
    }
    
    my %digits = (
        0 => '', 1 => 'one ', 2 => 'two ', 3 => 'three ', 4 => 'four '
+,
        5 => 'five ', 6 => 'six ', 7 => 'seven ', 8 => 'eight ', 9 => 
+'nine '
        );
    my %teens = (
        10 => 'ten ', 11 => 'eleven ', 12 => 'twelve ', 13 => 'thirtee
+n ',
        14 => 'fourteen ', 15 => 'fifteen ', 16 => 'sixteen ',
        17 => 'seventeen ', 18 => 'eighteen ', 19 => 'nineteen '
        );
    my %tens = (
        2 => 'twenty ', 3 => 'thirty ', 4 => 'fourty ',
        5 => 'fifty ', 6 => 'sixty ', 7 => 'seventy ',
        8 => 'eighty ', 9 => 'ninety '
        );
    my $groupStr = '';
    
    # Deal with last two digits
    my $tensStr = $group % 100;
    
    $groupStr = $tens{int ($tensStr / 10)} if $tensStr >= 20;
    
    if (int ($tensStr / 10) == 1) {
        $groupStr .= $teens{int $tensStr};
    } else {
        $groupStr .= $digits{$tensStr % 10}
            unless $tensStr > 10 and $tensStr % 10 == 0;
    }
    
    my $hundreds = int ($group / 100);
    
    if ($hundreds) {
        my $prefix = "$digits{$hundreds}hundred ";
        
        $prefix .= 'and ' if $groupStr;
        $groupStr = "$prefix$groupStr";
    }
    
    my $prefix = toWords ($leftBit, $groups + 1);
    
    $prefix .= qw(thousand million billion trillion)[$groups] . ' ' if
+ $prefix;
    
    $groupStr = "$prefix$groupStr";
    $groupStr = 'zero' unless $groupStr or $groups;
    return $groupStr;
}


__DATA__
0
1
10
11
20
21
99
100
901
98012785623123
[download]

Prints:

0 = zero
1 = one 
10 = ten 
11 = eleven 
20 = twenty 
21 = twenty one 
99 = ninety nine 
100 = one hundred 
901 = nine hundred and one 
98012785623123 = ninety eight trillion twelve billion seven hundred an
+d eighty five million six hundred and twenty three thousand one hundr
+ed and twenty three
[download]

looks recursive to me, but it would be easy to make it iterative instead (left as an exercise for the reader).