The CHECKand INITblocks run when compilation is complete, but before the program starts. CHECK can mean "checkpoint," "double-check," or even just "stop." INIT stands for "initialization." The difference is subtle: CHECK blocks are run just after the compilation ends, whereas INIT blocks are run just before the runtime begins (hence, the -c command-line flag to Perl runs up to CHECK blocks but not INIT blocks).

Perl calls these blocks only during perl_parse( ), which mod_perl calls once at startup time. Therefore, CHECK and INIT blocks don't work in mod_perl, for the same reason these don't:

panic% perl -e 'eval qq(CHECK { print "ok\n" })'
panic% perl -e 'eval qq(INIT  { print "ok\n" })'