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.

Sometimes I forget that "never" means "you probably don't want to". ;-) Next time I'm in that code, I'll make it the responsibility of the called function. Thanks.

$! was an unfortunate choice of variable. :-) It's a dualvar so it has different values if treated as a string or as a numerical.

open my $fh, '<', 'this does not exist';
printf "%s (%d)\n", $!, $!;

__END__
No such file or directory (2)
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lodin

The example was fine. It doesn't make sense for logger to need the dualvar.

• If you wanted to pass the error string to the sub, the caller would use "$!".
• If you wanted to pass the error number to sub, the caller would use 0+$!.
• If the sub needs both an error string and an error number, it would take two arguments.
• If the sub wants to look at $!, it wouldn't take it as an argument.