Perla literally translates to "pearl," a beautiful and precious gem. However, in this song, ROSALÍA uses the word with heavy irony to describe a person who is the exact opposite.
She sings, "Es una perla, una de mucho cuidado" (He's a pearl, one to be very careful with). This sarcastic use, similar to calling someone "a real gem" in English, turns a compliment into a warning about a difficult and untrustworthy person. It's a perfect example of how context and tone can completely change a word's meaning in Spanish.
Have you ever met someone who is just a complete and total disaster? ROSALÍA has, and she wrote a whole song about him. "La Perla" is a hilariously detailed list of every single thing that is wrong with this guy. She calls him everything from a "ladrón de paz" (peace thief) to a "terrorista emocional" (emotional terrorist), and even the "mayor desastre mundial" (biggest worldwide disaster). The song sarcastically calls him a "perla" (pearl), which in Spanish can mean a real piece of work, someone you have to be very careful with.
She gets so specific it's brilliant. This guy is apparently an expert at being a "bala perdida" (loose cannon) and his greatest achievement, his "masterpiece", is his "colección de bras" (bra collection). He's a "Red flag andante" (walking red flag) who will blame his bad behavior on his doppelgänger. The advice is clear: he's a champion of disappointment, so if you ever meet him, don't lend him anything because, as the song says, "no lo devolverá" (he won't return it).