Alternatively, if you want/need to keep the Math::BigInt object intact, you can use threads::shared::shared_clone:

use strict;
use threads;
use threads::shared;
use Math::BigInt;
use Data::Dump qw[ pp ];
use feature qw(say);

my @numbers = ( 1 .. 10 );

my %result : shared;

my @threads = map {
    threads->create( \&process, $_ );
} @numbers;

$_->join for @threads;

for my $key ( sort{ $a <=> $b } keys %result ) {
    say "$key : ", $result{ $key }, ' => ', $result{ $key }->bstr;
}

sub process {
    my $number    = shift;
    my $factorial = factorial($number);
    lock %result;
    $result{$number} = shared_clone( $factorial ); ## clone the object
}

sub factorial {
    my $number = shift;
    Math::BigInt->bfac($number);
}
[download]

Produces:

C:\test>junk
1 : Math::BigInt=HASH(0x3f5d160) => 1
2 : Math::BigInt=HASH(0x3f5d190) => 2
3 : Math::BigInt=HASH(0x3f5d148) => 6
4 : Math::BigInt=HASH(0x3f5d178) => 24
5 : Math::BigInt=HASH(0x3f5d118) => 120
6 : Math::BigInt=HASH(0x3f5d130) => 720
7 : Math::BigInt=HASH(0x3f5d160) => 5040
8 : Math::BigInt=HASH(0x3f5d190) => 40320
9 : Math::BigInt=HASH(0x3f5d148) => 362880
10 : Math::BigInt=HASH(0x3f5d178) => 3628800
[download]