The use of Inline::Files and \*STDOUT was simply for demo purposes; hence the "I'll leave you to replace Inline::Files and \*STDOUT with more appropriate I/O.".

I'm not sure whether you've got a handle on this. If you have, you probably won't need the following; if not, here's some hints and tips that may prove useful for your real code. Ask if you need more information, further explanation, etc.

I'd recommend the autodie pragma. This will save a lot of effort checking whether files could be opened, created, read, written and so on. Put it near the top of your code; I usually write this:

...
use strict;
use warnings;
use autodie;
...
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Remove the 'use Inline::Files; line.

Use open to create your filehandles.

The output filehandle ($out_fh) is very straightforward:

open my $out_fh, '>', '/path/to/output_file';
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The input filehandles are a little more complicated but still fairly easy. I wouldn't recommend creating all the filehandles in advance; instead, open and close one at a time. The \*FILE0, \*FILE1, etc. in my original code are filehandles for the Inline::Files: you'll see I didn't need to explicitly create those. Perhaps change:

my %doc_fh_for = (File0 => \*FILE0, File1 => \*FILE1, File2 => \*FILE2
+);
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to something like this

my %source_file_for = (File0 => 'filename0', File1 => 'filename1', Fil
+e2 => 'filename2');
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and then further down

for (@feature_files) {
    open my $in_fh, '<', $source_file_for{$_};
    write_xml_content($_, $in_fh, $out_fh, ' ' x 8);
    close $in_fh;
}
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I don't know whether you have a predetermined list of source files or if you have to find them in one or more directories; if the latter, you may find readdir useful for this. Also note that may need to prefix the filenames with directory paths: while simple concatenation may suffice, I'd normally use File::Spec as it's portable (it's also a core module so there's no need to install it).