The code is a query parser for the http POST method. $ENV{} refers to the process environment variable named in the braces. Http servers present POST data on the STDIN filehandle. The innermost block is intended to decode and parse the data. Details of parsing don't look quite right.

You need to use CGI.pm for this. The learning curve is not too steep and the code quality is exceptional.

After Compline,
Zaxo