$folder = $entryfolder->get(); #get the value from the entry widget
if ( $folder =~ /[\$\\*&.?\s]/ ) {
  $dialog = $sw-> Dialog(
                       -title=> 'Alert',
                       -text => "Special characters appear in the name of your folder.",
                       -buttons => qw(OK) )
    ->Show();
}

##</code><code>##

/[\$\\*&?\s]|^\./