Good idea - thanks.

I eventually did find a way to simplify the math, and here's the test case I sent off:

#!perl
use strict;
use warnings;
use Test::More tests => 4;
use bignum;

my $lnev = -7 / (10**17);
my $ev=exp($lnev);

is( sprintf('%0.5f',$ev)     , '1.00000', '($ev) is approx. 1'     );
is( sprintf('%0.5f',1-$ev)   , '0.00000', '(1-$ev) is approx. 0'   );
is( sprintf('%0.5f',1-"$ev") , '0.00000', '(1-"$ev") is approx. 0' );

cmp_ok( $ev, '!=', 0, '$ev should not equal 0');
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@/=map{[/./g]}qw/.h_nJ Xapou cets krht ele_ r_ra/;
map{y/X_/\n /;print}map{pop@$_}@/for@/
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