Negligence is unintentional, so at least for the first offence due to negligence, there should be no punishment or kammic results (imo). What of recurring negligence?
Content with that ...(see sutta)..., he does not exert himself further in solitude by day or seclusion by night. For him, living thus heedlessly, there is no joy. There being no joy, there is no rapture. There being no rapture, there is no serenity. There being no serenity, he dwells in pain. When pained, the mind does not become centered. When the mind is uncentered, phenomena do not become manifest. When phenomena are not manifest, he is reckoned simply as one who dwells heedlessly
See the whole chapter on Heedfulness (Dhp 21-32) on the danger of heedlessness and the benefit of heedfulness.