"Pick a Perk" isn't a standard Magic keyword or ability—it doesn't appear in the official comprehensive rules or on any printed cards. It may be a custom mechanic from a homebrew set, a casual house rule, or a name confused with another term.
If you encountered this on a card or in a playgroup, ask for the specific text, since the actual rules are defined by what's printed, not the label. Mechanics that "pick" from options usually work like modal abilities (similar to "choose one") or menu-style effects such as those on Adventure cards or planeswalker loyalty abilities, where you select a benefit when the ability resolves.
For accurate Commander rulings, check the card's Oracle text via Gatherer or Scryfall. If it's a custom card, confirm how your table interprets it before the game to avoid disputes mid-play.