I'm almost blind with this monitor, but this looks like a simple typo. Maybe you need " instead of ' around

my $param = param('para_$co');
[download]

so that it's

my $param = param("para_$co");
[download]

hve not tried it, this is just a first look.

While we're at it, I might suggest you to use a little more meaningful names and case discipline for your variables, too; it'll make debugging easier.