For what it is worth, I experienced this too. Suddenly most WHM functions were returning 500 Internal server error with "Premature end of script headers..." for most UI reachable functionality. Seems to be resolved now, though Perl seems to have regressed to an earlier version. I had to update manually.
I also received tye following email from cpaddons_report.pl
Code:
"Can't locate YAML/Syck.pm in @INC (@INC contains: /usr/local/cpanel /usr/local/cpanel/cpaddons /usr/local/cpanel /usr/local/cpanel/whostmgr/docroot/cgi /usr/local/cpanel/cpaddons /usr/lib64/perl5/5.8.5/x86_64-linux-thread-multi /usr/lib/perl5/5.8.5 /usr/lib64/perl5/site_perl/5.8.5/x86_64-linux-thread-multi /usr/lib64/perl5/site_perl/5.8.4/x86_64-linux-thread-multi /usr/lib64/perl5/site_perl/5.8.3/x86_64-linux-thread-multi /usr/lib64/perl5/site_perl/5.8.2/x86_64-linux-thread-multi /usr/lib64/perl5/site_perl/5.8.1/x86_64-linux-thread-multi /usr/lib64/perl5/site_perl/5.8.0/x86_64-linux-thread-multi /usr/lib/perl5/site_perl/5.8.5 /usr/lib/perl5/site_perl/5.8.4 /usr/lib/perl5/site_perl/5.8.3 /usr/lib/perl5/site_perl/5.8.2 /usr/lib/perl5/site_perl/5.8.1 /usr/lib/perl5/site_perl/5.8.0 /usr/lib/perl5/site_perl /usr/lib64/perl5/vendor_perl/5.8.5/x86_64-linux-thread-multi /usr/lib64/perl5/vendor_perl/5.8.4/x86_64-linux-thread-multi /usr/lib64/perl5/vendor_perl/5.8.3/x86_64-linux-thread-multi /usr/lib64/perl5/vendor_perl/5.8.2/x86_64-linux-thread-multi /usr/lib64/perl5/vendor_perl/5.8.1/x86_64-linux-thread-multi /usr/lib64/perl5/vendor_perl/5.8.0/x86_64-linux-thread-multi /usr/lib/perl5/vendor_perl/5.8.5 /usr/lib/perl5/vendor_perl/5.8.4 /usr/lib/perl5/vendor_perl/5.8.3 /usr/lib/perl5/vendor_perl/5.8.2 /usr/lib/perl5/vendor_perl/5.8.1 /usr/lib/perl5/vendor_perl/5.8.0 /usr/lib/perl5/vendor_perl .) at /usr/local/cpanel/Cpanel/DataStore.pm line 10.
BEGIN failed--compilation aborted at /usr/local/cpanel/Cpanel/DataStore.pm line 10.
Compilation failed in require at /usr/local/cpanel/Cpanel/cPAddons.pm line 11.
BEGIN failed--compilation aborted at /usr/local/cpanel/Cpanel/cPAddons.pm line 11.
Compilation failed in require at /usr/local/cpanel/whostmgr/docroot/cgi/cpaddons_report.pl line 10.
BEGIN failed--compilation aborted at /usr/local/cpanel/whostmgr/docroot/cgi/cpaddons_report.pl line 10."