syntax error with constant

chunlou has asked for the wisdom of the Perl Monks concerning the following question:

Perhaps a duh newbie question, but why will

use constant k => 2;
print 1-k;
print 1-k();
print 1 -k();
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cause syntax error, whereas

use constant k => 2;
print 1-&k;
print 1- k();
print 1- k;
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won't? Thanks.