Many different replies about something being wrong and yet if we break down into the ordered steps one should expect Perl or even C to take:

1) evaluate a list of two arguments 0) $x = 0 and 1) $x++ = 0.

2) increment $x so that the list of arguments is now 0: $x=1 and 1) $x++ = 0 (stays unchanged - that's what postincrement is for!)

3) Call the function with the two arguments whose values we just established should be 1 and 0 respectively.

Then we see that the results are the only results we should expect to happen!