Software Engineering and Computer Science (CS) programs often contain several mathematical courses or courses with large mathematics components or dependencies. Study success in those subjects can be influenced by students' attitudes towards mathematics and their perceptions about their own relation with mathematics. To gain insights into how the mathematics perceptions of computer science students affect their studies, we collected qualitative data from fourteen CS students from two universities in the context of an elective mathematics course. In this qualitative research, we used a triangulation strategy by collecting data from three different sources (interviews, questionnaires, and math-history assignments) to develop a comprehensive understanding of CS students' mathematics perceptions. The thematic analysis revealed the factors that affect students' perceptions about mathematics, including various types of prior experiences, their self-efficacy, math anxiety, and motivation sources. The analysis also highlighted that students find mathematics important for skill transfer to specific CS topics and for supporting continuous learning.