The perfect subjunctive tend to translate to may have. It is most often used for Subjunctive clauses (See Subjunctive Uses), in which case they resemble the present indicative when translated to English. These clauses include the purpose clauses, result clauses, indirect questions and commands, and time clauses.

For Example: She might have killed him. She might have wanted him dead.

moneo, monere, monui, monitus, warn:

To make a perfect active subjunctive, take the 3rd principle part (monui), add "-eri" to the end, and then add the appropriate endings (as seen below). Monueris would mean "You may have warned".

To make a perfect passive subjunctive, you take the 4th principle part (monitus), and then add the verb for to be. Monitus sim would mean "I may have been warned."