sub Init {
    use Common::Module;
    my ($common_var01, $common_var02) = ($foo, $bar);

    if ($^0 eq "MSWin32") {
        require Win32::Baz;
        require Win32::Quux;

        common_sub01 = "common_sub01_nt";
        common_sub02 = "common_sub02_nt";
    }
    else {
        require Foo;

        common_sub01 = "common_sub01_unix";
        common_sub02 = "common_sub02_unix";
    }
}

sub common_sub01_nt {
    ...
}

sub common_sub01_unix {
    ...
}

# main program
# (where we call the subs)
&$common_sub01($foo, $bar) if $foobar;