In mod_perl a lot of perl's CORE functionality is overridden to get mod_perl scripts to exhibit a different behaviour in the Apache environment, to enable persistence across subsequent invokations, faster execution times, etc.

Perl Specifics in the mod_perl Environment from the practical mod_perl book explains why it is necessary to sometimes use CORE functions to override functions defined in the mod_perl/Apache modules.

I'm not sure why open() isn't CORE::open() in your case as it shouln't be the case. Maybe it helps to explicitly call the intended function.

CORE::open STDOUT, '>', '/dev/null'  or ...
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