I've never been able to work out an algorithm for calculating exp for rationals.
That's probably because exp(r) is irrational for every rational r except for 0. However, saying use bigrat sprinkles some magic crazy dust over the whole program in a way similar to use bignum; witness: 
use bigrat;
use strict;
my $lnev = - 7 / (10 ** 17);
print "lnev is $lnev\n";
my $ev = exp($lnev);
print "ev is $ev\n";
print "1-ev is ", 1-$ev, "\n";

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Again, both values are 1.
--
@/=map{[/./g]}qw/.h_nJ Xapou cets krht ele_ r_ra/;
map{y/X_/\n /;print}map{pop@$_}@/for@/

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In reply to Re^7: Help me make a test case for Math::BigFloat by fizbin
in thread Help me make a test case for Math::BigFloat by fizbin

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