Why not experiment and see what happens? E.g.

A.pm

package A;

BEGIN { print "compiling A\n" }

print "executing A\n";

1;
[download]

Z.pm (Not B since there is a B module already)

package Z;

BEGIN { print "compiling Z\n" }

print "executing Z\n";

1;
[download]

script.pl

use strict;
use warnings;

BEGIN { print "compiling script.pl\n" }

use A;
use Z;

print "executing script.pl\n";
[download]

result of running script.pl

compiling script.pl
compiling A
executing A
compiling Z
executing Z
executing script.pl
[download]