The declaration's prototype has to match the definition's prototype.

sub THREAD_PRIORITY_BELOW_NORMAL();
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Alternative:

BEGIN {
    if ($^O eq 'MSWin32') {
        require Win32::Process;
        import Win32::Process;
    }
}

if ($^O eq 'MSWin32') {
    my $pid = Win32::Process::GetCurrentProcessID();
    if (Win32::Process::Open(my $obj, $pid, 0)) {
        $obj->SetPriorityClass(THREAD_PRIORITY_BELOW_NORMAL);
    }
}
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