I'd suggest you look at the timeouts section of the docs where it says:

Setting read_interval to 0xffffffff will do a non-blocking read. The ReadFile returns immediately whether or not any characters are actually read.
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In reply to Re: Help using a tied filehandle from Win32::SerialPort in IO::Select by BrowserUk
in thread Help using a tied filehandle from Win32::SerialPort in IO::Select by Anonymous Monk

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