tlm showed you how to get the (lone) value of a single parameter. To display all the parameters:

use CGI ();

my $cgi = CGI->new;

print("Content-Type: text/plain\n\n");

# Loop over each parameter.
foreach my $param_name ($cgi->param) {
   # Loop over each value of the parameter.
   foreach my $value (@{$cgi->param($param_name)}) {
      print "$param_name = $value\n";
   }
}
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