Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
pĕr-attentus, a, um, adj., very attentive: animus, Cic. Verr. 2, 3, 5, § 10.
Adv.: pĕrattentē, very attentively: audire aliquem, Cic. Cael. 11, 25.