Some serious instances of plagiarism have been discussed in Bangladesh's institutions, where students, researchers, and even faculty members were given severe penalties. This study explores the factors influencing teachers of English at the tertiary level in Bangladesh regarding plagiarism, including emerging concerns around AI-based plagiarism. A mixed-method research design has been used to answer two research questions: (1) What are the perceptions of English teachers about plagiarism at the tertiary level? and (2) What are the factors that influence these perceptions? Data were collected through a survey questionnaire administered to 128 English teachers from 25 public and private universities in Bangladesh, followed by in-depth interviews with 10 teachers involved in research supervision. Factor analysis revealed institutional negligence, lack of writing courses, absence of plagiarism policies, and infrequent use of plagiarism detection software as major drivers of plagiarism. The thematic analysis identified concerns relating to the misuse of AI tools that escalate plagiarism and make its detection difficult. Respondents expressed the need for guidelines with respect to the ethical application of AI in academic writing. Recommendations include making writing courses mandatory, including plagiarism policies in course outline, and implementing AI-specific plagiarism detection tools to strengthen the academic integrity of higher education.