One way to check for mod_perl is to check the error_log file for the following message at server startup:
[Sat May 18 18:08:01 2002] [notice] Apache/1.3.24 (Unix) mod_perl/1.26 configured -- resuming normal operations
 
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Written by Eric Cholet (Logilune) and Stas Bekman (StasoSphere & Free Books).