Also, likely, LD_LIBRARY_PATH must be set by the user that starts Apache.

Apache (and I suppose the IBM variant, too) normally comes with a file envvars, that you can use to set variables like LD_LIBRARY_PATH. The file is located in the .../bin directory of the Apache installation, and is being sourced from the apachectl start script.

@almut-I did find the envvars file and it did have the LD_LIBRARY_PATH defined. But, upon reinstall I did export the path found in envvars after starting the server just to make sure:

export LD_LIBRARY_PATH="/usr/IBM/HTTPServer/lib:/usr/IBM/HTTPServer/gs
+k7/lib:$LD_LIBRARY_PATH"
[download]

Install of mod_perl2 seemed to run fine, then got this prompt:

CPAN: YAML loaded ok (v0.68)

  CPAN.pm: Going to build G/GO/GOZER/mod_perl-2.0.4.tar.gz

no conflicting prior mod_perl version found - good.

Next we need to know where the 'apxs' script is located. 

Please provide a full path to 'apxs' executable
(press Enter if you don't have it installed):  /usr/IBM/HTTPServer/bin
+/apxs
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Install continues, a whole boatload of script runs past my eyes but I catch the following:

LD_RUN_PATH="/usr/IBM/HTTPServer/lib:/lib64" gcc  -shared -O2 -g -pipe
+ -Wall -Wp,-D_FORTIFY_SOURCE=2 -fexceptions -fstack-protector --param
+=ssp-buffer-size=4 -m64 -mtune=generic APR.o modperl_error.o modperl_
+bucket.o modperl_common_util.o modperl_common_log.o  -o ../../../blib
+/arch/auto/APR/APR.so        \
           -L/usr/IBM/HTTPServer/lib -laprutil-1 -ldl -lpthread -lexpa
+t -L/usr/IBM/HTTPServer/lib -L/usr/IBM/HTTPServer/lib -lapr-1 -lrt -l
+crypt -lpthread -ldl      \

/usr/bin/ld: skipping incompatible /usr/IBM/HTTPServer/lib/libaprutil-
+1.so when searching for -laprutil-1
/usr/bin/ld: skipping incompatible /usr/IBM/HTTPServer/lib/libaprutil-
+1.so when searching for -laprutil-1
/usr/bin/ld: skipping incompatible /usr/IBM/HTTPServer/lib/libaprutil-
+1.so when searching for -laprutil-1
/usr/bin/ld: skipping incompatible /usr/IBM/HTTPServer/lib/libexpat.so
+ when searching for -lexpat
/usr/bin/ld: skipping incompatible /usr/IBM/HTTPServer/lib/libexpat.so
+ when searching for -lexpat
/usr/bin/ld: skipping incompatible /usr/IBM/HTTPServer/lib/libexpat.so
+ when searching for -lexpat
/usr/bin/ld: skipping incompatible /usr/IBM/HTTPServer/lib/libapr-1.so
+ when searching for -lapr-1
/usr/bin/ld: skipping incompatible /usr/IBM/HTTPServer/lib/libapr-1.so
+ when searching for -lapr-1
/usr/bin/ld: skipping incompatible /usr/IBM/HTTPServer/lib/libapr-1.so
+ when searching for -lapr-1
chmod 755 ../../../blib/arch/auto/APR/APR.so
cp APR.bs ../../../blib/arch/auto/APR/APR.bs
...
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And then of course at the end, install abends:

/usr/bin/perl -Iblib/arch -Iblib/lib \
        t/TEST -clean
[warning] setting ulimit to allow core files
ulimit -c unlimited; /usr/bin/perl /root/.cpan/build/mod_perl-2.0.4-mN
+k2wE/t/TEST -clean
APACHE_TEST_GROUP= APACHE_TEST_HTTPD= APACHE_TEST_PORT= APACHE_TEST_US
+ER= APACHE_TEST_APXS= \
        /usr/bin/perl -Iblib/arch -Iblib/lib \
        t/TEST -bugreport -verbose=0
[warning] setting ulimit to allow core files
ulimit -c unlimited; /usr/bin/perl /root/.cpan/build/mod_perl-2.0.4-mN
+k2wE/t/TEST -bugreport -verbose=0
[  error] You are using mod_perl response handlers
[  error] but do not have a mod_perl capable Apache.
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Did you give it the correct source code that this server was built from? I believe I did point to the correct IBM directory for this. Looking at this problem I'm having, I have to ask is this an issue with Perl and the mod_perl2 install, or an IBM/Apached issue? Am I seeking help in the right forum? If not, I apologize, but do appreciate the help I've gotten here.

The "incompatible lib..." text implies you built a 64-bit mod_perl on a system where IHS is 32-bit. Try export CC="gcc -m32" (this can happen when the Apache build was created on a system where "gcc" is 32-bit and you build your mod wheree "gcc" defaults to 64-bit.