what does foreach do?

[Gary]

I hope this is the right place to ask this question.

So I've seen code like this

for (1..10) {
    print $_
}


What's going on behind the scenes with that? Is it creating an array? What if I 
want to use

for (1..1000000) ?

Also, What's the internal structure of Perl arrays? I can't find anything on 
that. I'm curious about how much time it takes to do something like insert into 
the middle ofan array. Is that O(1)?

Thanks