I presume that the problem is within Readonly, as reducing your program to a simple case without Readonly makes the problem go away:

package Cmds;
use strict;
use warnings;

sub RO(\[$@%]@) { warn "Assigning $_[0] <= $_[1]"; ${$_[0]} = $_[1] };

RO our $sig => "&#65285;&#65285;"; #improbable filename prefix
warn "\$sig lives at " . \$sig;

sub skey($) {$sig.$_[0]}

our %Global_Cache = (
    skey("asis")                =>    0,
);

package main;

print "Done\n";
[download]

I recommend you post actual, self-contained, working code.

As an aside, the string "％％" likely does not do what you think it does, unless you are outputting HTML.