my $i = 3;                             # from line 4 ...
while ($i < @lines - 2) {              # ... until last but 3
    if ($lines[$i] =~ m!<MYTAG></MYTAG>!) {
        splice(@lines, $i - 3, 6);     # kill 6 lines from $i - 3
    } else {
        $i++;
    }
}