convert very large unsigned numbers to base62?

I'd like to convert very large unsigned numbers (ala bigint) to a text
string using base62. I created this PL/PERL function to do the trick:

--------------------
CREATE OR REPLACE FUNCTION "public"."ls_crypt_convert_base" (in_value
text, in_base integer) RETURNS text AS
$body$
my ($value, $base) = @_;
$base = ($base > 62) ? 62 : (($base < 2) ? 2 : $base);
my @nums = (0..9,'a'..'z','A'..'Z')[0..$base-1];
my $index = 0;
my %nums = map {$_, $index++} @nums;

# short circuit if no value
$value =~ s/\D//g;
return if ($value == 0);

# this will be the end value.
my $rep = '';
while ($value > 0) {
$rep = $nums[$value % $base] . $rep;
$value = int($value / $base);
}
return $rep;
$body$
LANGUAGE 'plperl' IMMUTABLE RETURNS NULL ON NULL INPUT SECURITY INVOKER;
--------------------

# SELECT ls_crypt_convert_base(999999999999999999::text, 62);
ls_crypt_convert_base
-----------------------
1bS0EMtBbK8
(1 row)

# SELECT ls_crypt_convert_base(888888888888888888::text, 62);
ls_crypt_convert_base
-----------------------
13F7tmqjhmu
(1 row)

# SELECT ls_crypt_convert_base(7777777777::text, 62);
ls_crypt_convert_base
-----------------------
8umLiF
(1 row)

# SELECT ls_crypt_convert_base(123456789::text, 62);
ls_crypt_convert_base
-----------------------
8m0Kx
(1 row)

Did I just reinvent the wheel? It seems like something like this is
should already be built into PostgreSQL and I just don't know where to look.

-- Dante