A while back, I wrote Fun with join. I think it's great that you have a possible use for it. :-)

Alternatively, you could use List::Util 'reduce' to roll your own version of join that does the same thing.

use List::Util 'reduce';

my @arr = ('A'..'L');
my $per_line = 2;
my $on_this_line = 0;
print reduce {$a . (++$on_this_line % $per_line ? ',' : "\n") . $b} @a
+rr;
[download]

or

my @arr = ('A'..'L');
my $sep = [',', "\n"];
print reduce {$a . ($sep->[@$sep] = shift @$sep) . $b} @arr;
[download]

The last works, though it's possibly too clever. What if the left @$sep is evaluated before the shift?