Learn Polish: Usłyszałem anegdotę o młodym wędrowcu. - I heard an anecdote about a young wanderer.

You would say o owym młodym wędrowcu. Because the preposition o meaning "about" requires the locative case, the pronoun ów must also be in the locative masculine singular. It declines exactly like a standard adjective, taking the ending -ym to become owym.

Why does the locative form of wędrowiec end in -u (wędrowcu) instead of -ie?

Masculine nouns that end in a "hardened" consonant (such as -c, -cz, -rz, and -sz) take the -u ending in the locative case, because the -ie ending is reserved for consonants that can be softened. Additionally, notice that the "mobile e" from the nominative form disappears, changing the stem from wędrowiec to wędrowc- before adding the suffix.

Why use the perfective usłyszałem here instead of the imperfective słyszałem?

The perfective usłyszałem emphasizes a single, completed event: you were told the anecdote and caught the whole thing. If you used the imperfective słyszałem, it would imply a repeated action ("I used to hear this anecdote") or a state of having heard rumors about it without the finality of a single storytelling event.

Is wędrowiec a common word for a traveler in everyday conversation?