#!/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.