You have Alt::Crypt::RSA::BigInt installed (not Alt::Crypt::RSA)? Also install Math::BigInt::GMP.

One rather crude way to check is to do man Crypt::RSA and check the REPORTING BUGS section. If it has a third paragraph talking about segmentation faults and Math::Pari, then you've got the original Math::Pari version. Here's a more complicated but better one command line method:

perl -E 'use Crypt::RSA::Key; my($pari,$big) = ($Math::Pari::VERSION, 
+$Math::BigInt::VERSION); my $lib = Math::BigInt->config()->{lib} if $
+big; printf "Using %s\n", $pari ? "Pari $pari" : $big ? "BigInt with 
+$lib" : "???";'
[download]

With Crypt::RSA installed it shows Using Pari 2.01080900 where with Alt::Crypt::RSA::BigInt it shows Using BigInt with Math::BigInt::GMP

/dev/urandom shouldn't be hanging. Crypt::Random reads from /dev/random which can block waiting for more entropy. Alt::Crypt::RSA::BigInt version 0.06 (April 2017) should just read about 256 bytes at the start, as it uses an ISAAC CSPRNG after that.