Learn Spanish (Latin America): Al poner la carnada en el anzuelo, la perdí. - Upon putting the bait on the hook, I lost it.

Both are correct, but al + infinitive is a very common and efficient way to express "upon doing [something]." It emphasizes that the second action (la perdí) happened simultaneously or immediately after the first action.

Does the action in al poner have to be done by the same person who does the main action (perdí)?

Generally, yes. Because perdí means "I lost," it is automatically assumed that "I" am also the one putting the bait on the hook. If a different person was putting the bait on, you would usually switch to a full phrase like cuando él puso la carnada.

The translation says "on the hook", so why use en instead of sobre?

While sobre specifically means "on top of" or resting on a surface, en is the standard Spanish preposition for both "in" and "on" when referring to a general location, insertion, or attachment. Since bait is pierced or wrapped onto a hook, en is the most natural choice.

In la perdí, how do you know exactly what was lost?