"Ser un(a) perla" = To be a pearl*

Ironic expression to refer to someone who usually misbehaves. The expression "Ser un caso" carries a similar meaning and if the behaviour is extreme in a bad way, people say "Ser un caso perdido"(to be a lost case*)
 "Tiene un niño que es un perla"
"Cuidado con ella, que es una buena perla" 
"Los alumnos de ese grupo son un caso (perdido)".