You're not using param() correctly.
You have passed 'id=$id' to your CGI. Therefore, you can expect param('id') to return the value contained in $id.
If you had this in source.cgi:

$testo=qq(<a href="/cgi-bin/a_test/test.cgi?id=$id&fred=wombat">$testo
+</a>);
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Then you could do something like this:

foreach(param())
{
    print "$_ = ".param($_)."<br>\n";
}
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And you would get:

id = [whatever was in $id]
fred = wombat
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This should demonstrate the two ways to call param().