If you can't get rid of the recursion, you could always tighten up the program. If you create a mapping of characters to their next character, look how short your final subroutine becomes:

#!/usr/bin/perl -w
use strict;

my $number = '000';
my %mapping;
@mapping{ 0..9,'a'..'z' } = ( 1..9,'a'..'z','0' );

do {
    increment( \$number );
    print "$number\n";
} until $number eq 'zzzzzzzzzzzzzzzz'; # or some other long string

sub increment {

    my ( $number_ref ) = @_;
    my @digits = split //, $$number_ref;

    unshift @digits, undef while @digits < 16;
    _increment_digit( \@digits, @digits - 1 );

    $$number_ref = join '', grep { defined } @digits;

}

sub _increment_digit {
    my ( $array_ref, $index ) = @_;
    $array_ref->[$index] ||= 0;
    $array_ref->[$index] = $mapping{ $array_ref->[$index] };
    _increment_digit( $array_ref, ($index - 1) ) if $array_ref->[$inde
+x] eq 'z';
}
[download]

Cheers,
Ovid

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