• Case1: you're resetting $i each time at the beginning
• Case3 the $i scoped within the block is differentš from the outer one
  (update: and you are resetting the inner one each time again)

Generally better use prefix-constructs like

while () {...}, until () {...} or  for (;;) {...}

e.g. Case 3:

 
use strict;
use warnings;

my $i=42;
print "outside: $i\n";
for (my $i=5; $i>0; $i--) {
  print "inside: $i\n";
};
print "outside: $i\n";

my $j=666;
print "outside: $j\n";
{ my $j=5;
  do {
   print"inside: $j\n";
   $j--;
 } until($j<=0)
}
print "outside: $j\n";
[download]

prints:

outside: 42
inside: 5
inside: 4
inside: 3
inside: 2
inside: 1
outside: 42
outside: 666
inside: 5
inside: 4
inside: 3
inside: 2
inside: 1
outside: 666
[download]

As you can see, does the postfix construction force you to an extra block to limit the scope of the inner $j.

Update

expanded code example

Footnotes

1) see Coping with Scoping