it seems equally unnatural to write the body of fn() any differently (by making a copy of $_[0] before performing the substitution.

What are you talking about? Using one of my ($str) = @_; or my $str = shift; is standard practice, hardly unnatural.

If the issue is that you want both the original and the transformed string, you might also be interested in the following idiom: 

sub fn {
   my ($old) = @_;
   (my $new = $old) =~ s/.../.../;
   ...
}

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In reply to Re^3: Strange modification of @_ by ikegami
in thread Strange modification of @_ by jrw

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