% perl -wle 'print 3.9 * 3.9'
15.21
% export PERL5OPT='-Minteger'
% perl -wle 'print 3.9 * 3.9'
9
% export PERL5OPT='-Mbigint'
zen2% perl -wle 'print 3.9 * 3.9'
9
% export PERL5OPT=''
% perl -wle 'print 3.9 * 3.9'
15.21
%