my $var1;
my $var2 = undef;
my $var3 = "";
my $var4 = 0;
my $var5 = 1;
my $var6 = "foo";

for ($var1,$var2,$var3,$var4,$var5,$var6) {
  print defined($_)?1:0 . "\t" .
    ($_)?1:0 . "\n";
}

__END__
Output:
0   0
0   0
1   0
1   0
1   1
1   1