copy($groups_qfn, $backup_qfn)
    or die("Can't backup $groups_qfn\n");

tie(my @lines, 'Tie::File', $groups_qfn)
    or die("can not open $groups_qfn: $!\n");

for (@lines) {
    if (/^$group:/) {
        $_ = "$newstring\n";
        last;
    }
}

untie @lines;