open(FILE,">temp") or warn "error message" and $x = 1;
[download]

Or, if you don't feel a need to use one line:

open(FILE,">temp") or $x = 1;
$x and warn "error message";
[download]

Or, do what dreadpiratepeter said to, if you want to be absolutely clear. :)

- Matt Riffle
  Sr. Systems Programmer, pair Networks, Inc.
  (although, I speak only for myself; code is untested unless otherwise stated)