Mnemonics are "memory aids"; in GTK+, a mnemonic is an underlined character which corresponds to a keyboard accelerator. For a button, that means pressing Alt and that key activates the button.
Gtk2-Perl uses mnemonics by default on widgets that support them.
If characters in label string are preceded by an underscore,
they are underlined.
If you need a literal underscore character in a label,
use '__' (two underscores).
If you don't want to use mnemonics at all,
use the non-mnemonic version explicitly (e.g.