You could print out your pid.

print "my pid is $$ <br>\n";
[download]

I would suspect your pid would be the same as one of the httpd daemons if mod_perl were running correctly.

Update:

Also, from the mod_perl faq:

How can I test that my script is running under mod_perl?

There are 2 environment variables you can test.

exists $ENV{"MOD_PERL"}   # if running under mod_perl

$ENV{"GATEWAY_INTERFACE"} eq "CGI-Perl/1.1"

The MOD_PERL variable gets set immediately when the perl
interpreter starts up, whereas GATEWAY_INTERFACE may not be
set yet when BEGIN blocks are being processed.