I get the same failures on cygwin :( this should be filed as a serious bug. Too bad really, usually I use Math::Pari so I don't test bignum often, o just to test a new version of M::p

This is perl, v5.8.7 built for cygwin-thread-multi-64int (with 1 registered patch, see perl -V for more detail)

% stephan@labaule (/home/stephan) %
% perl -Mbignum -e 'print 10**-1.0'       
NaN
% stephan@labaule (/home/stephan) %
% perl -Mbignum -e 'print 10.0**-1.0'
NaN
% stephan@labaule (/home/stephan) %
% perl -Mbignum -e 'print 10**-1'     
NaN
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hth --stephan