my arg3=(defined($_2) : $_2 : 0;

Missing $?

You are commenting on modern perl. That already has something way better:

my $arg3 = $_2 // 0;
[download]

return unless my $line = <$fh>; to me is always preferable to my $line = <$fh> or return;

To me that is 1000% the other way around. Perception? Preference? Doesn't matter, as long as you are consistent.

If I have to maintain (as only maintainer) a piece of perl code, I will *rewrite* *all* statements as you state from action if expression; to expression and action; as that (to me) is waaaaaaaaaay easier to read/understand/maintain. Nothing to do with "idiomatic perl". Nothing at all!

In those case you should always use and/or and not &&/||:

$debug and warn "Line = '$line'\n";
$? > 0 and warn "...";
[download]