"Peruvian Necktie" refers to an arm triangle choke from the sprawl position, with one leg over uke's head and the other over their back:1
The IJF rules ban arm-triangles which "block the opponent's body with the legs", and alhough the example picture2 for this rule shows a do-jime/closed guard position (as opposed to the Peruvian necktie 'side-on' position), the wording is a big vague.
Is the Peruvian necktie competition legal?
Here it is being performed to success in the World Judo Championships 2021, which would seem to say "yes".
This term is also sometimes used to refer to the "Gerbi" choke, a similar choke applied with tori's gi-skirt.
Kata-sankaku grip situation in ne-waza: it is prohibited to block the opponent's body with the legs and mate must be announced (picture 2).