finally after trying everything I could imagine I gave up on compiling SAP::RFC on HP-UX 11.11.
I have no 32bit library /usr/lib/libdl.sl, only a /usr/lib/pa20_64/libdl.sl, therefore I
conclude I have to compile SAP::RFC with 64bit. Unfortunately this doesn't work either:
I downloaded the rfcsdk 6.40 HP-UX 64bit (PA-RISC) from the SAP Service Marketplace and put it in /usr/sap/rfcsdk.
/opt/perl_64/bin/perl Makefile.PL worked fine, it doesn't complain about missing libraries:
Checking if your kit is complete...
"make" works but shows some warnings: