In other words, you have a 128 bit number, and you want to represent it in base32.

my $binary_digest = ...;

my @digits = (0..9, 'A'..'V');
my $base32_digest = '';
for (0..3) { # 2^32^4 = 2^128
   $base32_digest = $digits[$binary_digest & 31]. $base32_digest;
   $binary_digest >>= 5;
}

print("$base32_digest\n");
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Update: Strike that. You don't start with a 128 bit number, it wouldn't fit. I'd reengineer my solution, but I must leave. Good luck.