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;

##</code><code>##

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

open(my $in_fh, '<', $backup_qfn)
    or die("Can't open $backup_qfn: $!\n");
open(my $out_fh, '>', $groups_qfn)
    or die("Can't open $groups_qfn: $!\n");

while (<$in_fh>) {
    if (/^$group:/) {
        $_ = "$newstring\n";
    }
    print $out_fh $_
}