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I have a Perl module with a template (for processing by the Template module) stored between the __DATA__ and __END__ keywords at the end of the file. When attempting to generate a file using the template, the resulting file comes out empty with no warnings or errors output. After debugging, I found that the DATA filehandle is actually empty before it is passed to the Template module.

A previous version of this module is able to correctly read the template from DATA, but none of the changes that I have made should be affecting this part of the code. These changes consist of logic changes within completely separate functions and adding the following use statements to the module:

use DBI;
use DBI::Const::GetInfoType;
use Switch;

I have tried adding write permissions on the perl module (it was originally read-only) and removing the __END__ keyword as I found that wasn't necessary. Unfortunately the DATA filehandle still appears empty.

What kind of problems could cause the DATA filehandle to be empty, and do any of these problems apply to my situation? I am using perl v5.12.5.

Rob Streeting
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    Note that the description for the `libswitch-perl` package in Debian stable says, "This is an obsolete module provided for compatiblity since it is being removed from the core. For perl 5.10 and above the `given`/`when` builtins are much preferred." Since you're using v5.12, those are available. – david Feb 10 '17 at 16:43

1 Answers1

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The reason that the DATA filehandle is empty in this case is down to the use of the Switch module. This module works by using a source filter which is clobbering the DATA filehandle during the course of its processing.

Alternatives include using if-elsif-else or using the given-when construct, although this is an experimental feature so it may not behave the same in later versions of Perl.

EDIT: Here is a simple reproducer for the issue described above: 

# use Switch;

while(<DATA>) {
  print($_);
}

__DATA__
One line of data
Second line of data

Without the "use Switch", you'll see the lines printed out, but with it nothing is printed.

Rob Streeting
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    I knew `Switch` was unpleasant but I didn't know it messed with your `DATA` section. Do you have a reference for that – Borodin Feb 10 '17 at 14:51
  • I added a reproducer to the original answer. I wasn't able to find specific reference to this issue online but the [source filter](http://perldoc.perl.org/perlfilter.html) documentation ("Things to look out for" section) mentions specifically that the DATA section can get clobbered by source filters. – Rob Streeting Feb 10 '17 at 16:53
  • Hmm. I guess it modifies data and code indiscriminately. Thanks very much. – Borodin Feb 10 '17 at 16:55