TIMTOWTDI :)

#!/usr/bin/perl -wl
use strict;
use Math::Fleximal;

my $b20 = Math::Fleximal->new("9998")
              ->change_flex([ map "<$_>", 0..19 ])
              ->to_str();
print "$b20\n";

# ...verbosely:
my $e = 0;
$e++ while $b20 =~ s/.*\K<(\d+)>/
                    my $m = 20**$e;
                    sprintf "%2d * %4d (20^%d) = %d\n",
                             $1, $m, $e, $1*$m /e;
print $b20;

__END__
<1><4><19><18>

 1 * 8000 (20^3) = 8000
 4 *  400 (20^2) = 1600
19 *   20 (20^1) = 380
18 *    1 (20^0) = 18
[download]

See also Math::Fleximal