First, don't process the QUERY_STRING by hand! Let CGI do that. For instance, parameters might get passed using ';' as a separator rather than '&'. See Ovid's CGI Course for more on that.

That said, the following would do

use CGI;
my $q = CGI->new;

my @list  = map { $q->param('xxx'.$_) } 1 .. $q->param('xxxc');
# values now sequential in @list.
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For not duplicating the values into @list, use

foreach my $index (1 .. $q->param('xxxc') ) {
    my $value = $q->param('xxx' . $index);
    # now do whatever with the value
    ...
}
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If you absolutely want your %INPUT hash, you get it populated with

%INPUT = $q->Vars;
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_($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}