Purpose: Despite substantial research on students' perceptions of collaborative writing, only a few studies have provided a deep understanding of their perceptions and attitudes toward computer-supported collaborative writing (CSCW). Previous studies have examined learners' perceptions and attitudes using questionnaires or interviews at a specific time, typically near the end of the study. Design: This mixed method study employed both qualitative and quantitative data and investigated 21 advanced English as foreign language (EFL) learners' perceptions of collaborative writing, and how these changed after using Google Docs (GD). It also elicited students' perceived affordances and constraints in using GD for either facilitating or hindering CW. The study used pre -- and post-questionnaires, semi-structured interviews, and students' online GD practice. Findings: The results revealed that using GD improved perceptions of CW and increased awareness of its benefits. Interestingly, the perceived challenges of CW changed significantly, indicating that GD may have addressed some of these challenges. Further, the analysis revealed that students perceived GD's pedagogical and technological affordances, but not its social affordances. While they appreciated how GD's sharing, editing, visualization, and feasibility features facilitated CW, they did not find it beneficial for social interaction. Several pedagogical implications will be discussed.