Note that the top level operator in the if() statement is an list-assignment. List-assignment returns the number of items on the right-hand side of the assignment. So only if mysub returns an empty list, the else part will be executed.

Now, you can get the value of $x by using:

if ((my ($x, $y) = mysub ($args))[0]) {
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but I don't think there's an easy way to say "mysub returns at least 2 values, or a non-zero value" while at the same time declaring and assigning those values to variables all in one go. But if you take the declaration out:

my ($x, $y);
if ((($x, $y) = mysub ($args)) != 1 || !defined($x) || length($x) != 1
+ || $x != 0) {
   do stuff
}
else {  # Triggered on a single 0
   do other stuff
}
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Whether this is the most readable solution, I'm not so sure.