You're asking about the first two stanzas of "A Receipt to Cure the Vapors" by Lady Mary Wortley Montagu:
Why will Delia thus retire,
And idly languish life away?
While the sighing crowd admire,
’Tis too soon for hartshorn tea:
All those dismal looks and fretting
Cannot Damon’s life restore;
Long ago the worms have eat him,
You can never see him more.
The whole poem uses a rhyme scheme known as "cross rhyme" or "alternating rhyme" -- ABAB CDCD EFEF, etc. In stanza 1, retire/admire is an example of perfect rhyme and away/tea is an example of slant rhyme. In stanza 2, fretting/eat him (probably pronounced "et him") is an example of slant rhyme and restore/more is an example of perfect rhyme.
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Korchnoi makes an excellent point. Pronunciations do change over time (as well as varying among different English-speaking populations), so one century's perfect rhymes can become another century's slant rhymes.