Note: try using m{...} instead of m/.../ anytime you want to have a regex that contains forward slashes. That way you don't have to escape them:

my $test = qq|
sdfiojs pfojsdfs fs [img]sdj fpsdofj spojf sfsf [b] [img]http://www.te
+st.com/image.gif[/img] dfs fs
s fsf sfd  [img]test.gif[/img]
|;

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