First, the code you posted has a syntax error in that you are missing a semi-colon after the first line below:

    $template_string = $_[0]
    %internal = %{$_[1]};
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There are also some stylistic oddities, such as using ALL CAPS for function names, calling the function with a leading &, and using prototypes (see next para), that caused me to pull a face -- though not errors they show dubious style IMHO.

Second, you probably shouldn't be using prototypes at all. For why not see:

• Perl Best Practices which, in Chapter 9, page 194 recommends: "Don't use subroutine prototypes".
• Far More than Everything You've Ever Wanted to Know about Prototypes in Perl -- by Tom Christiansen
• Modern Perl, "The Problem with Prototypes", page 242: Prototype coercions work in subtle ways, such as enforcing scalar context on incoming arguments ... (examples elided) ... Those aren't even the subtler kinds of confusion you can get from prototypes.

This new 5.12 warning that you experienced is described in perl 5.12 perldelta (search for "prototype after"). It seems you can silence this warning by adding the line:

no warnings 'illegalproto';
[download]

after your "use warnings" line (I am assuming you are using strict and warnings). However, I doubt that blindly silencing this warning is a good idea because it is telling you that the prototype is invalid -- presumably because of the optional arguments after the '%' character (I don't use prototypes so I'm not certain of that). See perlsub for Perl prototype documentation. You can also make it go away by simply removing the prototype (recommended) or shortening it to just:

sub POPULATE_TEMPLATE($%) {
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