use Data::Dumper;

my $data = str2data('foo[1].bar.baz[0]', 123);
print Dumper($data), "\n";

sub str2data {
    my($string, $value) = @_;

    my $tmp;
    my @idxs = ();

    for my $part (split /\./, $string) {
        if($part =~ /(.*?)\[(.*?)\]/) {
            push @idxs, $1, $2;
        } else {
            push @idxs, $part;
        }
    }
    while(defined(my $idx = pop @idxs)) {
        if($idx =~ /^\d+$/) {
            $tmp = [];
            $tmp->[$idx] = $value;
            $value = $tmp;
        } else {
            $tmp = {};
            $tmp->{$idx} = $value;
            $value = $tmp;
        }
    }

    return $value;
}