I might be tempted to use the ternery operator, but usually for a non-trivial assignment (like a join) I'd avoid it as it just gets messy. I'd probably do:

my $in;
if (ref $scalar eq 'ARRAY') {
   $in = join(",",@$scalar);
}
else {
   $in = $scalar;
}
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Not short, but pretty clear in what I mean (hopefully).