In order to use a conditional use, you'll need to place your code in a BEGIN block with a couple of evals. It's relatively easy:

#!/usr/bin/perl -l

use strict;
use warnings;

BEGIN {
       my $OS_win = ($^O eq "MSWin32") ? 1 : 0;
       print "Perl version: $]";
       print "OS   version: $^O";
                 
       if ($OS_win) {
           print "Loading Windows modules";
           eval "use Win32::SerialPort";
           die "$@" if $@;
       }
       else {
           print "Loading Unix modules";
           eval "use Device::SerialPort";
           die "$@" if $@;
       }
}
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