Checking if your kit is complete...
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Writing Makefile for Linux::NBD
cp lib/Linux/NBD/Client.pm blib/lib/Linux/NBD/Client.pm
cp lib/Linux/NBD.pm blib/lib/Linux/NBD.pm
cp lib/Linux/NBD/Server.pm blib/lib/Linux/NBD/Server.pm
/home/cpanrun/build/5.6.1/bin/perl -I/home/cpanrun/build/5.6.1/lib/5.6.1/i686-linux-thread-multi -I/home/cpanrun/build/5.6.1/lib/5.6.1 /home/cpanrun/build/5.6.1/lib/5.6.1/ExtUtils/xsubpp  -typemap /home/cpanrun/build/5.6.1/lib/5.6.1/ExtUtils/typemap NBD.xs > NBD.xsc && mv NBD.xsc NBD.c
Please specify prototyping behavior for NBD.xs (see perlxs manual)
gcc -c  -DUSE_REENTRANT_API -D_GNU_SOURCE -D_REENTRANT -fno-strict-aliasing -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64 -O2   -DVERSION=\"0.9\" -DXS_VERSION=\"0.9\" -fpic -I/home/cpanrun/build/5.6.1/lib/5.6.1/i686-linux-thread-multi/CORE  NBD.c
NBD.xs: In function `XS_Linux__NBD__Client__disconnect':
NBD.xs:48: `NBD_DISCONNECT' undeclared (first use in this function)
NBD.xs:48: (Each undeclared identifier is reported only once
NBD.xs:48: for each function it appears in.)
make: *** [NBD.o] Error 1