Depending on how much you can limit your input, one easy way would be to disallow [ in your URLs:

while ($test =~ m!\[img\]([^\[]+)\[\/img\]!g) {
   print "$1 \n";
}
[download]

If you really want to allow [ and ] in your tags as well, you can, I think, create some complex look-ahead regex that disallows for [img] to appear, by allowing for [ if it's not followed by i and allowing [i if it's not followed by m and allowing [im if it's not followed by g and so on:

while ($test =~ m!\[img\]((?:[^\[]+|\[[^i]|\[i[^m]|\[im[^g]|...)+\[\/i
+mg\]!g) {
   print "$1 \n";
}
[download]

Personally, I would restrict the input to disallow [ in URLs or just split on /\[img\]/ and then discard all strings that don't contain [/img].