Students have sung this at graduations for centuries, mostly without noticing it is a memento mori: rejoice while we are young, because after troublesome old age - the earth will have us. Whoever sharpened that drinking song, medieval clerk or Kindleben himself, knew what they were doing.
Grammatically it is the subjunctive's greatest hits. - let us rejoice - is the same hortatory form Adeste fideles drilled. , - long live - is the jussive, a wish aimed at someone else, and stanza four fires it four times. Pereat, pereant - may it perish - is the same form turned hostile, aimed at sadness, at the haters (osores), at the devil himself. One mood, three attitudes, and the tune hammers each ending onto its own beat.
One vocabulary flourish worth savouring: - "whatsoever you please" - survives in English as the name of a musical mash-up. The KU Leuven choir we link sings it as universities actually do, crisp and unhurried.