What lib are you using? How are you declaring accuracy and precision? Maybe this'll help:

#!/usr/bin/perl -l

BEGIN {
    $|  = 1;
    $^W = 1;
}

use strict;
use warnings;
use bignum lib => 'FastCalc';

use Memoize;
memoize('really_big');
really_big();

sub really_big {
    bignum->accuracy(15);
    bignum->precision(15);
    
    my $big_number = 3710728753390210279879799822083759024651013574025
+0
    + 46376937677490009712648124896970078050417018260538;
    print $big_number;
}
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