parse_uline() checks the string passed as the parameter for underscores, and then underlines the characters following the underscores. It will take the first underlined character in a label and return it as a lower-case accelerator key, e.g. _Save will return the accelerator key value for "s".
This method is used internally by Gtk+ for menu items.
Note that this method is deprecated in GTK+ 2.0.
$label->parse_uline("_File");