I've never used autouse, but I suspect you're having problems with the fact that use happens too early. You probably need to move them to a BEGIN block (which is where use "executes"). Something like this may work:

BEGIN {
    if ($^O eq "MSWin32") {
        use Win32::EventLog qw(Report new);
    } else {
        use Sys::Syslog qw(:DEFAULT setlogsock);
    }
}
# later...
    if ($^O eq "MSWin32") {
        my $eventlog = Win32::EventLog->new("myscript");
    } else {
        Sys::Syslog::setlogsockt ('unix');
        Sys::Syslog::openlog ("myscript", 'pid', "INFO");
    }
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HTH