That would be something like this, then.

my %checked = map { $_ => 1 } $qry->param("GRU$Za");
if (keys %checked > 1) {
    delete $checked{'E'};
} elsif (keys %checked == 0) {
    %checked = ('E' => 1);
}
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I'd still document it in the UI; it sounds like behaviour that is likely to cause confusion.