use strict;
use warnings;
use feature 'say';
{
    package Test;
    sub new {
        my $class = shift;
        my $ref = shift;
        return bless \$ref, ref $class || $class;
    }
    
    sub send {
        my $self = shift;
        my $data = shift;
        use Data::Dumper;
        say Dumper $$self; # ==> $VAR1 = \*{'::$file'}; so it is a GLOB ref
        print $$self $data;
        #the following does not warn...
        #~ my $fh = $$self;
        #~ print $fh $data;
    }
}

open my $file, '>', 'test.txt' or die $!;
my $t = new Test($file);
$t->send("foo");
close $file;

##</code><code>##

    sub output {
        my ($self, $data) = @_;
        print $$self $data;
    }