Not quite as sexy as BrowserUK's (though a bit clearer to me): here's my str2data.

sub str2data {
    my($string, $value) = @_;
    my $tmp;
    for my $part (reverse split /\./, $string) {
        undef $tmp;
        if ($part =~ /([^][]*?)\[(\d*?)\]/) {
            $tmp->{$1}[$2] = $value;
            $value = $tmp;
        } else {
            $tmp->{$part} = $value;
            $value = $tmp;
        }
    }

    return $value;
}
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It's single pass, but uses a tmp variable, which is somewhat clunky. Oh well.