tie *STDOUT, TrapClass;

print("foo\n");

my $pid = fork();
die "fork: $!\n" if !defined $pid;

if ($pid) {
   waitpid($pid, 0);
   print STDERR tied(*STDOUT)->get_buf();   # foo only
} else {
   print("bar\n");
}

package TrapClass;

sub TIEHANDLE {
    my $class = shift;
    my $buf = '';
    bless \$buf, $class;
}

sub PRINT {
    my $self = shift;
    $$self .= join($,, @_) . $\;
}

sub get_buf {
    my $self = shift;
    return $$self;
}