package UniWin;
BEGIN {
    if ( $^O =~ /^MSWin/ ) {
        require Win32::Setupsup;
        import Win32::Setupsup;
        @ISA = qw/Win32::Setupsup/;
    } else {
        require Expect;
        import Expect;
        @ISA = qw/Expect/;
    }
}

# Here you write a OS-generic wrapper around any OS 
# specific uses of Win32::Setupsup and Expect.  
# For example, if Setupsup has a "Win_This()" method and
# Expect has a "Unix_This()" method, you would write a
# wrapper called This() which invokes either method
# depending on which OS you're using...

1;

package main;
use strict;
use warnings;
use UniWin;
# ..........The rest of the program goes here...