Have you looked at Smart::Comments?

C:\test>perl
use strict;
use warnings;
use XML::Simple;
use Smart::Comments;

sub process_data{ return @_ }
sub send{
    ### Entering send...
    ### Exiting  send...
}

my $struct = XMLin('test.xml');
### $struct
if (defined $struct) {
    my $result_set = process_data($struct);
    ### $result_set
    &send($result_set);
}
^Z

### $struct: {
###            base => 'http://www.google.com'
###          }


### $result_set: 1


### Entering send...

### Exiting  send...
[download]

It even loads Data::Dumper for you, and to completely remove all the debug code, you simple comment out the use Smart::Commant; line. It does a whole lot more besides and avoids the problems with heredocs and the __DATA__ section.

I'm not keen on it retaining the ### marks in the output, and I would prefer a simple numeric argument to indicate which levels I want traced, but it's (IMO) the perfect use for source filters.