Untested code below. The trick? Pretend modules you don't want to load under certain conditions are already loaded.

BEGIN {
    if ($^O =~ /Win32/) {
        $INC{"Device/SerialPort.pm"} = 1;
    }
    else {
        $INC{"Win32.pm"} = 1;
        $INC{"Win32/TieRegistry.pm"} = 1;
        $INC{"Win32/SerialPort.pm"} = 1;
    }
}
use Device::SerialPort;
use Win32;
use Win32::TieRegistry;
use Win32::SerialPort;

.. rest of your logic ...
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Alternatively, use the if module:

use if $^O =~ /Win32/, "Win32";
use if $^O =~ /Win32/, "Win32::TieRegistry";
use if $^O =~ /Win32/, "Win32::SerialPort";
use if $^O !~ /Win32/, "Device::SerialPort";
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