Feasibility of ever completing $N=15 aside, the following is simple and uses next to no memory:

use Algorithm::Loops qw( NestedLoops );

my @chartab = (0..9, 'a'..'z');
my $N = 3;
NestedLoops(
    [ (\@chartab) x $N ],
    sub { print @_, ','; },
);
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