If $script::debug == "\0", then it will appear empty, but evaluate as true.
print "\Q$script::debug" would backslash all special characters, which would at least indicate their existence, though not their nature. TMTOWTDI, though there's probably a better way ;-)

Update:

my @unpack = unpack('C*', $script::debug);
print "Unpack info: @unpack\n";
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Should give you the ascii codes of each character in $script::debug, which will hopefully be useful.