After fixing the code a bit (I would assume that to have been typos), the following code works just fine for me:

use Tk;

$myForm = new MainWindow;

$angleEntry = $myForm->Entry(
    -width           => 12,
    -validate        => 'focusout',
    -validatecommand => \&angleCallback,
    -invalidcommand  => \&AngleError,
    -textvariable    => \$angle
)->pack;

$other = $myForm->Entry(-width => 12)->pack;

sub angleCallback
{
   my ($str, $newchar, $curstr, $index, $type) = @_;

   if (length($str) == 0 or $str !~ /^\d*.?\d*$/ or $str > 360)
   {
      $angleEntry->focus;
      $angleEntry->selection('range', 0, 'end');
      return 0;
   }
   return 1;
}

sub AngleError
{
   $myForm->messageBox(
      -title => 'Error',
      -message => 'Angle must be a number between 0.0 and 360.0',
      -type => 'OK'
   );
}

MainLoop;
[download]

Could you maybe provide a small script that reproduces your error?
CombatSquirrel.

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Entropy is the tendency of everything going to hell.