perl 'require' in begin block

Question

I have the following code:

#!/usr/bin/env perl
use strict;
use warnings;
use feature 'say';

BEGIN {
       my $supported = undef;
       *compute_factorial = sub { if (eval { require bignum; bignum->import(); 1;}) {
                                    my $num       = shift;
                                    my $factorial = 1;
                                    foreach my $num (1..$num) {
                                        $factorial *= $num; 
                                    }
                                    return $factorial;
                                  }  else {
                                       undef;
                                     } };
       };

my $f = compute_factorial(25);
say $f;

I'm just testing something, not really a production code... I do have bignum pragma on my machine (perfectly loadable using use), I was wondering why require doesn't work as it should be (I'm getting exponential numbers rather then "big numbers") in this case?

Thanks,

Answer 1

bignum's import needs to be called before compilation of the code it is intended to effect, or it doesn't work. Here, the BEGIN makes it called before your actual compute_factorial call, but not before the critical my $factorial = 1; is compiled.

A better approach for cases like this is just to directly use Math::Big*:

if (eval { require Math::BigInt }) {
    my $num = shift;
    my $factorial = Math::BigInt->new(1);
    foreach my $num (1..$num) {
        $factorial *= $num;                            
    }
    return $factorial;
} else {
    undef;
}