I've seen something very similar before, but it may be more about our environment - did you manually do the API installer or just rely on the NA product upgrade / install script (shown below):

(from upgrade or install script)

Please Wait

-----------

Installing PERL api ......
-

Now, I've not done that upgrade path, but this sounds like something we see.  For us, the fix has been one of two things:

Run the NA API installer manually after doing the NA upgrade or install (regardless of what is shown).  I can't recall what it is but seems like there is generally some underlying issue that the NA upgrade / install script misses or doesn't display but I see it when I manually install it and then I go to step 2 and fix whatever is shown.  

Check that all the modules are there and up to date.

Hope this helps.

thanks for the answer, do you have the procedure "Run the NA API installer manually" as well at hand?

I do, sort of....  We don't have that version and think the directories moved a bit, so I looked it up...I always just look for "har" and I find it.  :-)

Set up environment to install API components and program with API - Network Automation (microfocus.com)

Scroll down and you'll find this section:

Install NA Perl API

You can do the auto option (best case) or manual.  IMO, try the auto first and if that shows errors, you can switch to manual or just find what module it is complaining about, resolve the issue (usually a missing module it can't add itself) and then try again.  

perl client/perl_api/har/install.pl 

by chance, are you working with CentOS?  I see there's a link on problems with Perl API + CentOS

https://docs.microfocus.com/doc/Network_Automation/23.4/PerlAPIinstallscriptfailedinCentos

Good luck!