First -
source /opt/sap/SYBASE.sh ## This make sure the required {ENV} variables are set like $SYBASE etc

Second, i read somewhere that DBD::Sybase require $PERL5LIB to be set -
export PERL5LIB=$SYBASE/$SYBASE_OCS/perl/sybaseperl_64r/lib/lib:$SYBASE/$SYBASE_OCS/perl/sybaseperl_64r/lib/arch:$SYBASE/$SYBASE_OCS/perl/sybaseperl_64r/lib