in reply to Building Net::SFTP on Solaris problem.

GMP was compiled 64-bit. Your Perl is 32-bit. Recompile GMP for 32-bit integers (or you can recompile perl for 64-bit). Consult GMP documentation on how to do it.

Update: The option is ABI=32, i.e., you should reconfigure GMP by

./configure ABI=32
This yields
gmp-4.1.2$ file .libs/
.libs/        ELF 32-bit MSB dynamic lib SPARC32PLUS Version 1, V8+ Required,
UltraSPARC1 Extensions Required, dynamically linked, not stripped