You could log it first, and replace STDIN with a in-memory file (open(local *STDIN, '<', \$raw_data)) but the following might be sufficient:

use URI::Escape qw( uri_escape_utf8 );

foreach my $key ($cgi->param()) {
   foreach my $val ($cgi->param($key)) {
      printf("%s=%s\n",
         uri_escape_utf8($key),
         uri_escape_utf8($val),
      );
   }
}
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You could also look at CGI's save method.