use Data::Dumper;
my $allow_loose6 = 1;
my $aref = [1, 6, 2, 2, 7, 1, 6, 99, 99, 7];
my @new = ();
my @loose = ();
my $flag = 0;
for (@$aref) {
    $flag = 1 if (!$flag and $_==6);
    if (!$flag) {
	push @new, $_;
    }
    else {
        push @loose, $_;
    }
    if ($flag and $_==7) {
        $flag = 0;
	@loose = ();
    }
}
$aref = \@new;
if ($allow_loose6) {
    push @$aref, @loose;
}
elsif (@loose) {
    die "Unterminated 6 detected";
}
print Dumper $aref