What's the difference?

The original code had my $z = $z + 1;, this code has $z = $z + 1;

Like huck already said, "The extra my commands isolated the variables outside of the loop from any changes but it also meant the inner variables became almost static".

Using my $z you declare a second variable named $z, which shadows the first variable named $z declared at the top of your script. All statements mentioning $z will refer to that shadowing $z until execution leaves the block for the next loop iteration. In the next iteration, a new shadowing $z is created and all changes made previously have been lost.

use strict; use warnings; 
{
  print "using exterior values\n"; 
  my $y=1; 
  my $z=1; 
  my $hoeveel=5; 
  for (my $i=1; $i <= $hoeveel; $i++) {
     $y = $y + 155;
     $z = $z +1;
     print "$y $z\n"; 
  };
}

{
  print "using localizing interior values\n"; 
  my $y=1; 
  my $z=1; 
  my $hoeveel=5; 
  for (my $i=1; $i <= $hoeveel; $i++) {
     my $y = $y + 155;
     my $z = $z +1;
     print "$y $z\n"; 
  };
}
[download]

Result

using exterior values
156 2
311 3
466 4
621 5
776 6
using localizing interior values
156 2
156 2
156 2
156 2
156 2
[download]

I did what you did and left out the "my" but it didn't work. I get the same values for $y and $z.

Look where the $y and $z scalars are located. They are in the $a scalar. The values in the $a scalar needs to be changed.