I finally got Sybase ASE 15 installed. I used CentOS 4.3 under VMWare Server beta 3 (also found how to disable debugging):

As root:

yum -y update (enabled centosplus and contrib)
sysctl -w kernel.shmmax=67108864 (also add line to /etc/sysctl.conf, this doubles the shared memory to 64K)
useradd -d /opt/sybase sybase (creates a sybase user and sets its $HOME)
mv /usr/lib/tls /usr/lib/tls.bak (stops Sybase linking against the wrong Kerberos libraries)

As the newly-created user sybase:

cd /opt/sybase/installer
sh setup (developer and express worked, full install, accept defaults)
sh SYBASE.sh (sets $SYBASE and $SYBASE_OCS so DBD::Sybase can find client libraries)
cd /opt/sybase/installer/DBD-Sybase-1.07
perl Makefile.PL (specified VMCENTOS43 database and other defaults)

As root again:

make install
mv /usr/lib/tls.bak /usr/lib/tls

DBD::Sybase doesn’t seem to be able to connect yet, but isql works, and by adding jConnect.jar and jisql.jar to $CLASSPATH, I got jisql to work too (screenshot).

I found this howto, and created a new user and password.