sub doit{
     return "How are you?\n";
    }
    open (FILE, ">files.txt");
    print FILE &doit;
    close(FILE);
[download]

Or

    sub doit{
     my $handle = shift;
     print $handle "How are you?\n";
    }
    open (FILE, ">files.txt");
    doit(*FILE);
    close(FILE);
[download]

Or .. lots of ways.