rename renames, not copies.

Oops! My error. I did
print $fh_in $_;
where I meant to do
print $fh_out $_;

Both
my @lines = <$fh_in>; for (@lines) { s/// }
and
while(<$fh_in>) { s/// }
work a line at a time. The only difference is that the top version needlessly keeps the entire file in memory.

Entire file at once would be

my $text; { local $/; $text = <$fh_in>; }
$text =~ s///g;
[download]