Make sure 'use warnings' is NOT in YaBB.pl and there is no '-w' or '-W' in the shebang. Add
no warnings qw(uninitialized once redefine);
to all the .pm and .pl files just below the copyright block. That MAY stop the warnings you're seeing. I have no idea what changes on the server may be causing YaBB to be throwing warnings to the YaBB error.log except that what's in the log tells me that warnings is turned on somehow.
For the other errors:
in MediaCenter.pm find
@media_in = split /\&/gxsm, $media_in;
and change it to
@media_in = split /\&/xsm, $media_in;
(Don't know when that error crept in
.)
The epoch time error looks like a language support issue. In BoardIndex.pm find
if ( ${ $uid . $curboard }{'lastposttime'} ne 'N/A'
and replace with
if ( ${ $uid . $curboard }{'lastposttime'} =~ m{\A[0-9]+\Z}xsm
(We're changing looking for NOT 'N/A' to making sure the field only contains numbers.) I'm sure there are a lot more issues related to language support.
Unfortunately, I really have no idea what change on your server is triggering warnings. I do know it's not due to the Perl version.