BELOW_NORMAL_PRIORITY_CLASS and its sister ABOVE_NORMAL_PRIORITY_CLASS were introduced at Windows NT 5.0, sorry, Windows 2000. A lot of Win32 code is built for the more generice NT 3.5.

From the Win32::Process Changes file (on CPAN):
0.13  Fri Jun 13 2008

    ...
    - Add ABOVE_NORMAL_PRIORITY_CLASS and BELOW_NORMAL_PRIORITY_CLASS
      constants (not exported by default).

    ...

[download]
So it looks like you need a later version for these constants. I have not tried this, but it is possible you could declare your own:
BELOW_NORMAL_PRIORITY_CLASS       0x00004000
ABOVE_NORMAL_PRIORITY_CLASS       0x00008000

[download]

In reply to Re: How to lower my own priority on Windows? by Anonymous Monk
in thread How to lower my own priority on Windows? by rovf

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