What I was hoping to do was break it into separate "plug-ins" if you will that will only get loaded/run on specific platforms.

Try something like the following

if ( $^O =~ /win32/i ) {
    require MySysInfo::Win32;
    import MySysInfo::Win32;
} elsif ( $^O =~ /amiga/i ) {
    require MySysInfo::Amiga;
    import MySysInfo::Amiga;
} elsif # etc.
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If there's a possibility that the required module might not be present, you can wrap the require in an eval.

The import gives you a chance to export a common API into the top-level script's namespace (you'll need to implement an import sub in each of your packages -- examples of that abound).

Enumerating $^O is left as an exercise.