Provided your Perl was compiled using 'use64bitint', then you can do the following:

my $rand64 = (int(rand(4294967296)) << 32) | int(rand(4294967296));
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However, Math::Random::MT::Auto provides random 64-bit integers directly (again, provided your Perl is compiled using 'use64bitint'). Just install it from CPAN, and then:

use Math::Random::MT::Auto qw/irand/;

my $rand64 = irand();
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A lot simpler. Plus, the speed is about the same on Windows, and is a lot faster on Solaris.