$str = '
blah blah blah blah
blah
blah blah blah [blah
[blah blah] [blah blah blah
blah] blah]
blah blah
blah blah blah blah
blah
blah blah blah [blah
[blah blah] [blah blah blah
blah] blah]
blah blah
blah blah blah blah
blah
blah blah blah [blah
[blah blah] [blah blah blah
blah] blah]
blah blah
blah blah blah blah
blah
blah blah blah [blah
[blah blah] [blah blah blah
blah] blah]
blah blah';

##</code><code>##

        my $re = qr{\[(?:(?>[^\[\]]+)|(??{$re}))*\]}s;
        for (;;) {
                last unless $tempstr =~ s/(\[\w+?\s*=\s*($re|\n|[^\[\]])+\])/&assign($1)/gies;
        }