Can you post minimal code needed to reproduce the error? Also, what version of Win32::SerialPort and Perl are you using?

Looking at some old code of mine that used Win32::SerialPort, my code did do something along the lines of $serial->baudrate($baudrate); and it did work.

Out of curiosity, did you happen set the value of $baudrate like $baudrate = "38400";? If so, try changing that to be $baudrate = 38400; instead.