warn should include both the details, the position in the script/module, and the position in the input:

#! /usr/bin/perl
use warnings;
use strict;

print "" while <DATA>;
warn 'here';

__DATA__
1
2
3
[download]

Stderr:

here at /home/choroba/1.pl line 6, <DATA> line 3.
# in the script:      ~~~~~~~~~~~
# in the input:                    ~~~~~~~~~~~~~
[download]