I think the '???' is just the default string printed "for the value of a constant that can't be determined because it was optimized away" (the latter is quoted from the docs).  You can change it with option -sv, e.g.

$ perl -MO=Deparse,-sv"'UNKNOWN'." 698666.pl
use constant ('DEBUG', 0);
use warnings;
use strict 'refs';
print "Hello World\n";
'UNKNOWN';
print "Goodbye Cruel Word\n";
698666.pl syntax OK
[download]