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    $__to =~ s/(\r\n|\n\r|\r|\n)/\r\n/sg;
    $__cc =~ s/(\r\n|\n\r|\r|\n)/\r\n/sg;
    ...

my %mail =
(
...
    "Subject"         => "$__subject",
    "Content-Type"     => $__content_type,
);

for ( $__to, $__subject, $__content_type, ... ) {
    tr/\r\n/ /;
...
    "Subject"         => $__subject,
    "Content-Type"     => $__content_type,
);

my @message_lines = 
  ( '', # blank line
...
   );

my $_mail_message = join( "\r\n", @message_lines );