The & needs escaping as %26 like this:

mycode.pl?param=first&second=name&details=items%26things&other=more<br
+>
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There are modules to escape stuff for you. For example, URI::Escape -- this example is basically straight from the man page:

perl -MURI::Escape -le 'print uri_escape("10% is enough\n");
# 10%25%20is%20enough%0A
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