In that case, I pretty much answered this question yesterday.

From a file handle:

binmode $fh_in;
binmode $fh_out;

local $/ = \90;
while (<$fh_in>) {
   print $fh_out "$_\x0D\x0A";
}
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From a string:

binmode $fh_out;

while (length($str)) {
   my $rec = substr($str, 0, 90, '');
   print $fh_out "$rec\x0D\x0A";
}
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If you want CRLF when run on Windows and LF when run on other OSes, get rid of the binmode and replace \x0D\x0A with \n.