Well, I don't know if it's what you intended, but you gave me a clue to solving the problem - I was making my job much harder than it needed to be by constantly trying to handle transitions from Mandelbrot scale to canvas scale + edges to centre...

y concentrating on tracking the center I was able finally to get my head around what was needed... below is the working code - you can pass it a param of canvas size (e.g. "200"), or just let it use the default of 100.

Thanks all.

use strict;
use Tk;

my $c_size = ($ARGV[0] =~ /^\d+$/ && $ARGV[0]>0) ? $ARGV[0] : 100;
my $scale = my $oldscale = 1;
my ($lasti, $lastr)=(0,-0.5);

my $mw = MainWindow -> new;
my $canvas = $mw -> Canvas(-height=>$c_size, -width=>$c_size) -> pack;
$canvas -> Tk::bind('all', '<ButtonPress-1>' => \&MakeMan);
MakeMan();

MainLoop;

sub MakeMan(){
  $canvas -> delete('all');
  my ($e, $canv_x, $canv_y)=(0,$c_size/2,$c_size/2);
  if($scale > 1){
    $e = $canvas -> XEvent;
    $canv_x = $e->x;
    $canv_y = $e->y;
  }

  my $px_old = (4/$oldscale)/$c_size;
  my $px_new = (4/$scale)/$c_size;
  
  my $i = $lasti = $lasti + ($canv_y - ($c_size/2))*$px_old; # centre
  $i = $i - (($c_size/2)*$px_new); # left edge
  my $rconst = $lastr = $lastr + ($canv_x - ($c_size/2))*$px_old; # ce
+ntre
  $rconst = $rconst - (($c_size/2)*$px_new); # top edge

  my $icount = 1;
  while($icount <= $c_size){
    my $r = $rconst;
    my $rcount = 1;
    while($rcount <= $c_size){
      my $iter = ($c_size/10)*($scale**0.5);
      my $tr = $r;
      my $ti = $i;
      my $rgb_out = '#000000';
      my $rgb = 255;
      for my $n(1..$iter){
        ($tr, $ti) = (($tr ** 2) - ($ti ** 2) + ($r), (2 * $tr * $ti) 
++ ($i));
        $rgb -= int(255/$iter);
        if((($tr * $tr) + ($ti * $ti)) > 4){
          $rgb_out = '#' . (sprintf("%2.2X", $rgb))x3;
          last;
        }
      }
      $canvas -> createLine($rcount, $icount, $rcount+1, $icount, -fil
+l => $rgb_out, -width => 1);
      $rcount ++;
      $r += ((4/$scale)/$c_size);
    }
    $icount ++;
    $i += ((4/$scale)/$c_size);
  }
  $oldscale = $scale;
  $scale = $scale * 2;
}
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map{$a=1-$_/10;map{$d=$a;$e=$b=$_/20-2;map{($d,$e)=(2*$d*$e+$a,$e**2
-$d**2+$b);$c=$d**2+$e**2>4?$d=8:_}1..50;print$c}0..59;print$/}0..20
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