Perl does not have a history of disallowing things on the grounds of "readability"

Yeah, so it's odd that it's done here. But that was the reason given by Larry Wall (as I remember things).

I suspect they must lead to parsing ambiguities

Well yeah. Is X MODA Y MODB Z equiv to ( X MODA Y ) MODB Z or X MODA ( Y MODB Z )? But that the same applies to every binary operator. These ambiguities are resolved using associativity and precedence.