This study investigated the influence of L1-specific constraints on L2 perception. In particular, we examined the effect of the unreleased coda constraint in L1 on Korean learners’ perception of word final palatal sibilants across two proficiency levels, i.e., low vs. high. Our findings from a perception experiment showed that Korean learners, especially the low group learners, tended to hear the illusory vowel /i/ after a word final palatal sibilant. Thus, the low group learners had a difficulty in discriminating the contrasts of /ʃ/ vs. /ʃi/, /ʧ/ vs. /ʧi/, and /ʤ/ vs. /ʤi/. According to the perception experiment, the low group learners’ illusory vowel perception rates increased as frication noise of the word final palatal sibilant got longer. Additionally, the low group learners were not influenced by the relative duration of the vowel /i/ in perceiving the vowel. Rather, their vowel responses were more likely to be influenced by the frication duration of a word final palatal sibilant. The study revealed that L1 constraints are significant factors influencing L2 perception.