require v. use

[Connie Chan]

As what I've learnt, 'use' is working during the compile time, 
where 'require' works at runtime, however, I have problem on 
understand this..

### code ###
sub A
{    my ($case) = shift;
        if ($case == 0) { require "lib0.pl" ; use somePm; use anotherPm ; &doSth(...) }
        if ($case == 1) { require "lib1.pl" ; use somePm; use anotherPm ; 
&doSthEls(...) }
        if ($case == 2) { require "lib2.pl" ; use somePm; use anotherPm ; 
&doSthAnotherEls(...)}
}

sub B
{    my ($case) = shift;
      #  something similar to A
}

sub C
{    my ($case) = shift;
      #  something similar to A
}

### EOF Code ###

Say, if above is called by require, then I think it probrably those inline reqire and 
use
are calling when the sub is called and when the case is matched... however, what
about the above finally becomes a .pm would those use, require will be loaded once
I use the above pm ? 

I do really concern about the loading of system... so which is a better method for 
lower
the loading ?  I think exporter can help to lower some loading, but how about those....
Sorry,  I don't know how to phrase the question... =), unless I have some more clear 
concept on what is the real practise of runtime and compile time... 

Thanks in advice,
Connie