The documentation for
WWW::Mechanize says
tick and
untick accept the following parameters:
$mech->tick( $name, $value [, $set] )
$mech->untick( $name, $value )
I therefore suspect that what you really need to do is the following:
$self->{'agent'}->untick('newsletters', 17);
$self->{'agent'}->untick('newsletters', 65);
$self->{'agent'}->untick('newsletters', 34);
$self->{'agent'}->untick('newsletters', 71);
Or more succiently
$self->{'agent'}->untick('newsletters', $_) for (17, 65, 34, 71);
However, not knowing the form or the html that you're accessing, I can't say for sure what you're doing incorrectly. Can only go on the way it looks.