In Perl, arguments are passed by reference, not by value. This is what perlsub means when it says: “The array @_ is a local array, but its elements are aliases for the actual scalar parameters.”
The subroutine call a($x && $y) causes the Perl interpreter to evaluate the boolean expression $x && $y to determine which variable is to be aliased. If $x is false, the expression short-circuits, so a reference to $x is passed in to the sub; but if $x is true, short-circuiting does not occur, and so it is a reference to $y that is passed in to the sub.
So it is misleading to say that the boolean expression works as an l-value. Within sub a, the boolean expression is never seen — only the reference to either $x or $y — and it is this reference which works as an l-value.
I haven’t found anything in the documentation to explain the process by which Perl derives a reference (“alias”) from a complex expression given as an argument to a subroutine. It might be useful to know — but, in general, it’s safer and clearer to keep things simple and just pass a variable or a value.
Hope that helps,