Or you can use Algorithm::Combinatorics module, specifically variations_with_repetition function. It uses iterator approach, so it consumes quite little memory. As was noted above, the number of results is very large, so following prints just first 50:

use Algorithm::Combinatorics qw(variations_with_repetition);

my $iter = variations_with_repetition([ '0'..'9', 'a'..'z' ], 15);
while($seq = $iter->next) {
    print @$seq,"\n";
    last if ++$count > 50;
}
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-- Roman