Quote

perldoc Tk::Entry
(Sektion: VALIDATION)

      The validateCommand and invalidCommand are called with the
      following arguments:

      * The proposed value of the entry.  If you are configuring
      the entry widget to have a new textvariable, this will be
      the value of that textvariable.
      * The characters to be added (or deleted). This will be
      "undef" if validation is due to focus, explcit call to
      validate or if change is due to "-textvariable" changing.
      * The current value of entry i.e. before the proposed
      change.
      * index of char string to be added/deleted, if any. -1
      otherwise
      * type of action. 1 == INSERT, 0 == DELETE, -1 if it's a
      forced validation or textvariable validation

$mw->Entry(-validatecommand => sub { length $_[0]  <= $maxlength},