Some possibilities:

use if $^O eq 'MSWin32', 'Win32::SerialPort';
use if $^O ne 'MSWin32', 'Device::SerialPort';
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use if 1, $^O eq 'MSWin32
   ? 'Win32::SerialPort'
   : 'Device::SerialPort';
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BEGIN {
   eval(
      $^O eq 'MSWin32
         ? 'use Win32::SerialPort; 1'
         : 'use Device::SerialPort; 1'
   ) or die $@;
}
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