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use List::Util 'reduce';

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

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