If you're mostly just copying the contents of several files, perhaps consider not using a module at all. Something along the lines of this code (following some Notes) might do what you want.

Notes:

• You didn't specify exactly which parts of the source files were to be copied. I'll leave you to make changes depending on your requirements.
• IDs should be unique. I've added code to do this. Again, modify how you want.
• I've added consistent indentation. You may want something different: change to suit.
• Assumed a typo: s{<doc/>}{</doc>}
• I've used a cut-down version of your data for demo purposes.
• I'll leave you to replace Inline::Files and \*STDOUT with more appropriate I/O.

#!/usr/bin/env perl -l

use strict;
use warnings;

use Inline::Files;

my %doc_fh_for = (File0 => \*FILE0, File1 => \*FILE1, File2 => \*FILE2
+);
my $out_fh = \*STDOUT;
my $key_file = 'File0';
my @feature_files = qw{File1 File2};

print $out_fh '<Key="1234">';
write_xml_content($key_file, $doc_fh_for{$key_file}, $out_fh, ' ' x 4)
+;
print $out_fh '    <Status="in use"/>
    <Features>';
for (@feature_files) {
    write_xml_content($_, $doc_fh_for{$_}, $out_fh, ' ' x 8);
}
print $out_fh '    </Features>
    <Other_Status/>
</Key>';

sub write_xml_content {
    my ($file_id, $in_fh, $out_fh, $indent) = @_;
    
    while (<$in_fh>) {
        chomp;

        if (/^<\/?doc>$/) {
            s/doc/$file_id/;
        }
        else {
            if (/^<ABC id="(\d+)">$/) {
                my $id = $1;
                s/$id/$file_id-$id/;
            }
            $_ = ' ' x 4 . $_;
        } 

        print $out_fh $indent, $_;
    }
}
[download]

Inline::Files data:

Output:

---

---

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