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

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In reply to Re: use constant and Compiler Optimizations by almut
in thread use constant and Compiler Optimizations by jbisbee

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