I suspect you want this instead

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

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]

It turns out that it wasn't obvious what you wanted to do here after all. :-)

The problem (which you haven't spelled out) is that the here doc interpolates when you set $a so your initial $y is never changed. One solution is not to interpolate but to use a placeholder scalar (which could just be "$y" again) and then eval the here document.

#!/usr/bin/env perl 
use strict;
use warnings;

my ($y, $out);

$y = 'baby';
my $hd = <<'EOT';
Hello $y!
EOT

eval "\$out = qq{$hd}";
print $out;
$y = 'world';
eval "\$out = qq{$hd}";
print $out;
[download]

But that's a bit crude. Far better to use a proper templating system.

#! /usr/bin/perl
use strict;
use warnings;

print "Hoeveel?\n";
my $hoeveel = <STDIN>;
chomp $hoeveel;
print "You typed: $hoeveel\n";

sub xml {
    my ($y, $z) = @_;
    return << "EOT";
 <g
     transform="translate(-124.85653,$y)"
     id="layer$z">
    <text
       xml:space="preserve"
       style="font-style:normal;font-variant:normal;font-weight:normal
+;font-stretch:normal;font-size:126.0375061px;line-height:125%;font-fa
+­mily:Arial;-inkscape-font-specification:'Arial, Normal';text-align:s
+tart;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anch
+or:start;fill:#000000;fill-opacity:1;stroke­:none;stroke-width:1px;st
+roke-linecap:butt;stroke-linejoin:miter;stroke-opacity:1"
       x="-45.714272"
       y="235.21936"
       id="text3336"><tspan
         id="tspan3338"
         x="-45.714272"
         y="235.21936">a</tspan></text>
  </g>

EOT
}

my $y = -931.96483;
my $z = 1000;

for (my $i=1; $i <= $hoeveel; $i++) {
   $y += 155;
   ++$z;
   print xml($y, $z);
};
[download]

huck's advice about not redeclaring the variables inside the loop holds.

($q=q:Sq=~/;[c](.)(.)/;chr(-||-|5+lengthSq)`"S|oS2"`map{chr |+ord
}map{substrSq`S_+|`|}3E|-|`7**2-3:)=~y+S|`+$1,++print+eval$q,q,a,
[download]

It's clearly SVG, and re-using the id values for the text and tspan elements may or may not cause trouble.

Hello, WisDomSeeKer34.

It is obvious what I want to do here: I want to enlarge $y with 155 and $z with 1 by $hoeveel times. But how to do it?

The solution seems obvious:

$y += (155 * $hoeveel);
$z += (  1 * $hoeveel);
[download]