sub _defined {
   if (defined($_[0])) {
      return $_[0];
   } else {
      return $_[1]->();
   }
}

sub _or(&) {
   return $_[0];
}

my $pid = _defined fork(), _or { die "Can't fork: $!" };
[download]

I'll let you pick better names.

Update: Nevermind. If you try to pick useful names, you end up back with

my $pid = defined_or fork(), sub { die "Can't fork: $!" };
[download]