You need to specify the 'g' option to get all occurences, but sometimes relying on $1, etc. is overkill:

my $a = "this is a test [[1]] [[ a ]] [[ abd ]] [[\%object,key]]";
print $_,$/ for $a =~ /(\[\[[^\]]+\]\])/g;

__DATA__
[[1]]
[[ a ]]
[[ abd ]]
[[%object,key]]
[download]

UPDATE: Haven't you people read Death to Dot Star!? ;) Seriously, while a non-greedy dot star works does work for this example, i cannot recommend using it when there is a better solution: a negated character class ([^\]]+).

jeffa

L-LL-L--L-LL-L--L-LL-L--
-R--R-RR-R--R-RR-R--R-RR
B--B--B--B--B--B--B--B--
H---H---H---H---H---H---
(the triplet paradiddle with high-hat)