Actually you don't need to worry how big is big at all, just let Perl do everything for you.

Here is a piece of code I come up with: 
use Data::Dumper;

$str = $ARGV[0];
$a = $str + 0.0;
if ($a =~ m/INF/) {
    use bignum;
    $a = $str + 0.0;
    no bignum;
}

print Dumper($a);

[download]
Play with it like this: 
perl -w test.pl 9e5000, will give you a BigInt
perl -w 12.3, will give you a normal guy
With limited testing, I cannot promise that it will work for all the situations, so try it out.

In reply to Re: Determining when Math::BigFloat is necessary? by pg
in thread Determining when Math::BigFloat is necessary? by jasonk

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