Throw a "my" declaration in the loop statement:

for (my $i=0; $i<5; $i++) {
  print "$i : first_string ref[$i] second_string parent_ref.ref[$i] th
+ird_string\n";
}
[download]

Update:

Forgot the $ref2 bit. So:

my $ref2 = 'parent_ref';

for (my $i=0; $i<5; $i++) {
  print "$i : first_string ref[$i] second_string $ref2.ref[$i] third_s
+tring\n";
}
[download]