Some things just a little bit off.

#!/usr/bin/perl

# http://perlmonks.org/?node_id=1212198

use warnings;
use strict;
use Tk;

my $top = new MainWindow;
my $canv;
# offset use to decide the square around a given point
my $offset = 4;
my $num_of_points = 4;
my @points;   # ([x1,y1],[x2,y2]...)
my @tk_points;  # (tk-canvas,..)

$top->Label(-text => "How many points?")->pack(-side => 'left');
$top->Entry(-width => 3,-borderwidth => 4, -textvariable => \$num_of_p
+oints)->pack(-side => 'left'); 
$top->Button( -padx=> 5,
        -text => "draw",
        -borderwidth => 4, 
        -command => sub{&draw($num_of_points)})->pack(-side => 'left')
+;

$top->Button( -padx=> 5,
        -text => "clear",
        -borderwidth => 4,
        -command => sub{@points = ();
            map{ $canv->delete($_) }@tk_points;
            })->pack(-side => 'left');


# new toplevel for the circle
$canv=$top->Toplevel(-title=>'Points in a cirlce')->Canvas(-background
+=>'gray',
  -width => 610, 
  -height => 610)->pack;

# the big cirlce
my $circle = $canv->createOval(10,10,600,600,
               -fill => 'red', 
    -outline => 'yellow',
    -width => 5,
               -tags =>['circle'],
               -stipple => 'gray12',
          );

# draw the center at 305 305
fat_dot(305,305);

MainLoop;

sub fat_dot {
  # center of the new fat dot
  my ($x,$y) = @_;
  # canvas and top left and bottom right coords of the 
  # rectangle where the cirlce will be draw
  my ($x1,$y1,$x2,$y2) = ($x-$offset,$y-$offset,$x+$offset,$y+$offset)
+;
  my $dot = $canv->createOval($x1,$y1,$x2,$y2,-fill => 'black');
  return $dot;
}

sub draw{
  my $num = shift;
  print "received $num as num of points\n";
  # sin and cos they think in rad not in degrees!!
  my $ang = 3.141592653589793238462643383279 * 2 / $num;
  print "angle: $ang radiants\n";
  my $cur = 0;
  for (1..$num){
    my $x = 305 + int(295*sin($cur));
    my $y =  5 + 300 - int (295*cos($cur));
    print "at $cur radiants point at: $x $y\n";
    my $dot = fat_dot($x,$y);
    $cur+=$ang;
    push @points,[$x,$y];
    push @tk_points,$dot;
  } 
}
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