Then whats the point in separating compile time and run time in Perl6? In a way so as long there is a BEGIN block isn't that impossible.

In the general case it's impossible to "separate" compile time and run time, yes. In Perl 5 as well as 6.

But please remember that BEGIN and &eval aren't there because one day we looked the other way and they just snuck in. They're there because sometimes we need them as escape hatches when what we want to do can't be done by less crude means.

Then whats the point in separating compile time and run time

Speed. There's no reason to compile code every time it's executed. That would be very slow, and there's no advantages to doing it that way.

In a way so as long there is a BEGIN block isn't that impossible.

Not at all. It just prevents the compile-time of all code blocks from happening at the same time.

Speed

And consistency.

Consider code like

for (1..10) {
    # some statements
  
    # grammar modification here
}
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If compile time and run time modification were not separated, the head of the loop would be once parsed without the grammar modifications, and then reparsed with grammar modifications - a nightmare for readability and debugging.

Perl 6 - second systems done right

I had mentioned code polymorphism in my reply, but I removed it before posting because code polymorphism can still occur with a single compile-time. Yes, it's nightmarish, but it's not a reason for separating compile-time.