You mentioned playing with $/, but what about $\ (output sep)?

If you're trying to write compatible files (did I understand that correctly?), then why use \n at all? Why not just explicitly write out the EOL string? Like this:

sub plp_writeln {
    # plp_writeln( $HANDLE, @elem );
    # writes @elem joined as one line to the file $HANDLE for .plp
    my $HANDLE = shift;
    my @elem = @_;
    
    print $HANDLE, @elem, qq[\x{0D}\x{00}\x{0A}\x{00}];
}
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Or, similar idea, subbing all newlines for your custom newline:

sub plp_write {
    # plp_write( $HANDLE, @elem );
    # writes @elem to the file $HANDLE for .plp, interpolating newline
+s
    my $HANDLE = shift;
    my @elem = @_;
    
    ( my $str = join('',@elem) )=~s[\n][\x{0D}\x{00}\x{0A}\x{00}]gs;
    print $HANDLE, $str;
}
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