The problem is not with use constant:

sub k{}
print 1-k;
print 1 -k();
print 1 -k();
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gives the same error. This happens from at least 5.6.0 until 5.8.1-RC4.

What I think happens is that the parser gets confused with trying to handle the:

 -key => value
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notation.

Liz

Update: -k  File has sticky bit set. is of course a much better explanation. Thanks to BrowserUk!