This is what the README has to say about the .enc files:

These maps were generated by a perl script that comes with the module
XML::Encoding, compile_encoding, from XML formatted encoding maps that
are distributed with that module. These XML encoding maps were generat
+ed
in turn with a different script, domap, from mapping information conta
+ined
on the Unicode version 2.0 CD-ROM. This CD-ROM comes with the Unicode
Standard reference manual and can be ordered from the Unicode Consorti
+um
at http://www.unicode.org. The identical information is available on t
+he
internet at ftp://ftp.unicode.org/Public/MAPPINGS.
[download]

so, my guess would be that you have to run your 8859-15.TXT file through these two scripts...

Or, take the easy way out, and redeclare your XML input to be in ISO-8859-1 :)

ISO-8859-1 is pretty close to ISO-8859-15, but you wouldn't have the Euro sign, for example. See ISO-8859-15 for an exact list of differences.  Depending on what the end output is of your processing chain (i.e., if it's still in ISO-Latin* single-byte encoding, and not something like UTF-8), it might work if you simply redeclare it back to ISO-8859-15 when done.

___

Side note: please don't use <pre>...</pre> tags - at least not for long lines (use <c>...</c> instead). Most browsers don't auto-wrap those lines, but add horizontal scrollbars instead — in short, the page gets messed up... (Your "<pre>This function looks... </pre>" in Re^3: Error with .xml file processing in XML::Simple is definitely too long for <pre>, and some people would even say that 20 chars is 'long').