You probably want to append ':constant' to your use Math::BigInt declaration (see the section entitled "Autocreating Constants" in the documentation). But it doesn't look like you are doing integer math, so maybe BigFloat is more appropriate. Try...
#!/usr/bin/perl -w
use strict;
use Math::BigFloat ':constant';
my ($out,$diver,$grade) = (2.84894533078051,5,0.002);
my $in = Math::BigFloat->new(($out)**(1/$grade)*$diver);
print $in;