$fo ||= 'Roses are red ...';
print "My homework: $fo";

##</code><code>##

$ perl -E '
    my $fo = "";
    update($fo);
    print "|$fo|";

    sub update {
        my ($arg) = @_;
        $arg .= "XXX";
    }
'
||

##</code><code>##

$ perl -E '
    my $fo = "";
    update(\$fo);
    print "|$fo|";

    sub update {
        my ($arg) = @_;
        $$arg .= "XXX";
    }
'
|XXX|