That's something most people will never need to know.

The problem is that new programmers love to put *everything* in quotes. While it's a bad idea to pass global (lexical and package) variables as arguments, that is probably not a problem you'll ever run into. Just about every sub already creates a copy of the value before it can change.

sub logger {
   my ($arg) = @_;   # The copying happens here, so
   ...               #   no need to do it in the caller.
}
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One could even argue it's the sub's responsibility to protect global variables it changes if it uses @_ at any other point.