There have to be exceptions to every rule :-). This was obviously not the problem with the present question though. And, I am inclined think this is perhaps a little bug:

sub blah {
    print shift;
}
blah($s ++) for 1 .. 9;   # works fine
blah $s ++  for 1 .. 9;   # syntax error

print $s ++  for 1 .. 9;   # syntax error
print($s ++) for 1 .. 9;   # syntax error
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The latter print() case is probably related to print()'s prototype for optionally accepting a filehandle).