The speaker wakes up frightened, but not for the reason we would expect. Most people, the poem says, would assume a nightmare caused the fear. Instead, what scares the speaker is a dream that was simply too perfect.
That twist is the heart of the poem. A perfect dream usually feels like a gift, yet here it leaves the speaker shaken. The poem doesn't spell out why, so we are left to guess. Perhaps the dream showed a happiness that real life can't match, and waking up made the gap painful. Perhaps it felt too good to be trusted, like something that couldn't last. Either reading fits the short, plain lines.
The mood is quiet and unsettled. There is no dramatic imagery, just a calm confession, which makes the fear feel more honest. The title, in quotation marks, adds a hint of doubt about whether anything can really be "perfect." The poem leaves us with that uneasy thought.