Coerce Moose attribute from Num to Math::BigFloat (untested).

gistfile1.pl

use Moose;
use Moose::Util::TypeConstraints;
use Math::BigFloat;

subtype 'BigFloat',
    as class_type('Math::BigFloat');

coerce 'BigFloat',
    from 'Num',
    via { Math::BigFloat->new($_) };

has 'amount' => (
    is     => 'rw',
    isa    => 'BigFloat',
    coerce => 1,
);