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

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

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

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