It's not clear to me from your code, but I think you need to call $father_end->get() to retrieve the contents of the father entry box.

Style-wise, I also recommend not using global variables -- instead passing variables to your subroutines:

sub save_from_entry_widget
{
    my ($widget, $filename) = @_;
    my $text                = $widget->get();
    open my $fh, '>>', $filename or die "Cannot append to '$filename':
+ $!\n";
    print $fh $text;
}
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Then call this with: