Ant. Animated, spirited, assiduous, careful, ardent, zealous, interested.
Perhaps. By chance, peradventure, perchance, maybe, haply, possibly.
Ant. Certainly, inevitably.
Peril. Hazard, risk, jeopardy, danger, venture, insecurity, liability.
Ant. Safety, assurance, certainty, security.
Period. Time, cycle, age, epoch, limit, end, conclusion; era, duration, date, term, continuance, termination, conclusion, bound.
Ant. Eternity, infinity, perpetuity, endlessness, everlasting.
Periodical. Recurring, regular, stated, recurrent, systematic, calculable, serial.
Ant. Indeterminate, eccentric, fitful, spasmodic, irregular.
Permanent. Enduring, abiding, changeless, constant, durable, steadfast, unchangeable, immutable, invariable, fixed, unchanging, stable, lasting, persistent, perpetual.