"One Perfect Rose" sounds like a classic love poem—at first. The speaker begins by telling readers of a lover who sent her a "single flow'r," a blossom he selected with great care, "tenderly" choosing it to show the speaker his affection.
The antiquated spelling of "flow'r" gives the beginning of the poem a fancy, old-timey feel: this is the kind of spelling one would find in a classic love poem. But "One Perfect Rose" was published in 1926, long after poetry had moved past archaic spellings—and long after it had become a cliché to uses roses as symbols of love. The language choices here make it clear the speaker isn't being totally sincere as she describes her lover's romantic gesture.
That hint of sarcasm only grows as the speaker describes the rose her lover sent her:
Deep-hearted, pure, with scented dew still wet—
One perfect rose.
The words "deep-hearted" and "pure" imply that this lover thinks the rose is full of profound romantic meaning—and highlight his over-earnest, over-sincere, and actually pretty dull view of romance. In other words, the lover has romanticized romance itself, clearly thinking that giving the speaker a flower is some grand and deeply significant gesture. But the very idea of "one perfect rose" is a shallow cliché, and suggests that this guy's "love" might be similarly shallow.
The speaker's meter makes her cynical point even clearer. The first three lines are in iambic pentameter and the last line is in iambic dimeter. This means that lines 1 through 3 ("A single [...] still wet—") each contain five iambs, or metrical feet consisting of an unstressed syllable followed by a stressed syllable (da-DUM). Line 4, on the other hand, only contains two iambs (two da-DUMs). This isolates the phrase "one perfect rose," calling attention to it and spotlighting the speaker's point: it's kind of ridiculous to hang so much meaning on so dull and obvious a gift.