(my @v = keys %foo) == 1 or die "Got more than one: @v\n";
  print "V:@v\n";

##</code><code>##

Possible unintended interpolation of @v in string...
Global symbol "@v" requires explicit package name...

##</code><code>##

 my $x = parse_thing();
 my $y = $x->{item} or die "Item not found in x.\n";
 my @z; (@z = do_xyz()) < 3 or die "Got extra Z: @z\n";
 print "Z:@z\n";