Well, as I said, you can pass an instance of A (which is an object) to B and B can then call your the object's WriteLog method. You probably would want to pass the instance to B's constructor and allow B to contain the instance of A so it could use it wherever it needed it.

If A is a class implementing a log file and WriteLog() is a method to write an entry to, say, an open logfile managed by an instance of A and you need to write to the same log file from both the main and B packages, then this approach probably makes sense. Just make sure you document that B uses an instance of A to write to a log.

Update: Here is some example code:

#!/usr/bin/perl -w 
use strict;

package A;

sub new {
    my $class = shift;
    my ($fh)  = @_;
    bless { FH => $fh }; 
}

sub WriteLog {
    my $self = shift;
    my $fh = $self->{FH};
    print $fh "[",scalar localtime,"] ",@_,"\n"
}


package B;

sub new {
    my $class    = shift;
    my ($logobj) = @_;
    bless { A => $logobj };    # B contains instance of A.
}

sub Foo {
    my $self = shift;
    my $log = $self->{A};
    $log->WriteLog("B::Foo wrote this!");
}


package main;

my $A = A->new(\*STDOUT);
my $B = B->new($A);        # Pass instance of A to B's constructor.

$A->WriteLog("main wrote this!");    # main uses instance of A.
$B->Foo();                           # B also uses it.
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-sauoq
"My two cents aren't worth a dime.";