Useful links:
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perl html->xml special characters conversion
Convert HTML symbols to equivalent Unicode
http://www.perlmonks.org/?node_id=757360

Perl Binding for libxml2  
http://search.cpan.org/~shlomif/XML-LibXML-1.80/

Encode or decode strings with HTML entities
http://search.cpan.org/~gaas/HTML-Parser-3.68/lib/HTML/Entities.pm

HTML Tutorial
http://www.w3schools.com/html/default.asp

Elements of FB2 standard (rus)
http://www.fictionbook.org/index.php/%D0%AD%D0%BB%D0%B5%D0%BC%D0%B5%D0%BD%D1%82%D1%8B_%D1%81%D1%82%D0%B0%D0%BD%D0%B4%D0%B0%D1%80%D1%82%D0%B0_FictionBook