The POSIX function strtol provides a straightforward method of converting from a Base36 number. Unfortunately, there isn't an equivalent ltostr.

Here is an un-golfed solution. It edits the number in-place.

There is a optimisation at the beginning that uses a simple string increment for numbers that don't end in 9 or Z. This gives a speed-up for 17/18 of the Base36 range.

#!/usr/bin/perl -w

use strict;
use POSIX 'strtol';

my @chars = (0..9, 'A'..'Z');

sub inc36 {
    my $num = $_[0];

    if ($num !~ /[9Z]$/) {
        $num =~ tr/0-9/a-j/;
        ++$num;
        $num =~ tr/a-j/0-9/;
        return $_[0] = $num;
    }

    my $int = 1 + strtol($_[0], 36);

    my $base36 = '';

    while ($int) {
        my $frac = $int % 36;
        $int     = int ($int / 36);
        $base36  = $chars[$frac] . $base36;
    }
    $_[0] = $base36;
}
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--
John.