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" : "???";'
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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.

My perl instances seems to be using Math::BigInt::GMP

#/opt/perl/bin/perl -E 'use Crypt::RSA::Key; my($pari,$big) = ($Math::
+Pari::VERSION, $Math::BigInt::VERSION); my $lib = Math::BigInt->confi
+g()->{lib} if $big; printf "Using %s\n", $pari ? "Pari $pari" : $big 
+? "BigInt with $lib" : "???";'
Using BigInt with Math::BigInt::GMP
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However, when run the following using an RSA public key

my $ct = $pgp->encrypt( Filename => 'plain.txt', Recipients => $key, C
+ipher => 'DES3' ) or die "Encryption failed: ", $pgp->errstr;
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I see Pari library being opened.

#truss ./encrypt_openpgp.pl 2>&1  | grep open | grep Pari
open64("/opt/perl-5.26.0/lib/site_perl/5.26.0/sun4-solaris-thread-mult
+i/Math/Pari.pm", O_RDONLY) = 4
open("/opt/perl-5.26.0/lib/site_perl/5.26.0/sun4-solaris-thread-multi/
+auto/Math/Pari/Pari.so", O_RDONLY) = 4
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And when I see my code hanging it always has this file open: /opt/perl-5.26.0/lib/site_perl/5.26.0/Crypt/Random.pm

If I remove Math::Pari he program fails with

./encrypt_openpgp.pl
No random source available! at /opt/perl-5.26.0/lib/site_perl/5.26.0/C
+rypt/OpenPGP/Util.pm line 111.
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 sub get_random_bytes {
103         my $length = shift;
104         if (eval 'require Crypt::Random; 1;') {
105                 return Crypt::Random::makerandom_octet( Length => 
+$length);
106         }
107         elsif (eval 'require Bytes::Random::Secure; 1;') {
108                 return Bytes::Random::Secure::random_bytes($length
+);
109         }
110         else {
111                 die "No random source available!";
112         }
113 }
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I think i figured out what was happening. I didn't have Bytes::Random::Secure, so OpenPGP was still calling Crypt::Random which still uses Pari I removed and rebuilt OpenPGP. Now I don't see any references to the Pari library I'll see if this fixes my hanging issue