I came back to this, and found that if I perform the following actions, things *seem* to work out ok after I manually download, unpack and cd into the distribution directory:

perl Makefile.PL.solaris
gmake
gmake test
gmake install
[download]

Here is the standard Makefile.PL, that would be used by default when installing with cpanm:

use ExtUtils::MakeMaker;
 
$CC = 'g++';
 
%args = ('CCFLAGS' => '-Wall');
 
if($^O eq 'cygwin') {
  $args{'LDDLFLAGS'} = '-shared -L/usr/local/lib';
}
 
WriteMakefile('NAME'          => 'Algorithm::SVM',
              'VERSION_FROM'  => 'lib/Algorithm/SVM.pm',
              'PREREQ_PM'     => {},
              ($] >= 5.005 ?
               (ABSTRACT_FROM => 'lib/Algorithm/SVM.pm',
                AUTHOR        => 'Matthew Laird <matt@brinkman.mbb.sfu
+.ca>') : ()),
              'OPTIMIZE'      => '-O3',  # segfaults with gcc 2.96 if 
+lower (?)
              'LIBS'          => '-lm',
              'CC'            => $CC,
              'LD'            => '$(CC)',
              'OBJECT'        => 'SVM.o libsvm.o bindings.o',
              'XSOPT'         => '-C++ -noprototypes',
              %args);
[download]

...and here's the Makefile.PL.solaris file I used:

use ExtUtils::MakeMaker;
 
$CC = 'g++';
 
%args = ();
 
if($^O eq 'cygwin') {
  $args{'LDDLFLAGS'} = '-shared -L/usr/local/lib';
}
 
WriteMakefile('NAME'          => 'Algorithm::SVM',
              'VERSION_FROM'  => 'lib/Algorithm/SVM.pm',
              'PREREQ_PM'     => {},
              ($] >= 5.005 ?
               (ABSTRACT_FROM => 'lib/Algorithm/SVM.pm',
                AUTHOR        => 'Matthew Laird <matt@brinkman.mbb.sfu
+.ca>') : ()),
              'OPTIMIZE'      => '-O3',  # segfaults with gcc 2.96 if 
+lower (?)
              'LIBS'          => '-lm',
              'CC'            => $CC,
              'LD'            => '$(CC)',
              'CCCDLFLAGS'    => '-fPIC',
              'LDDLFLAGS'     => '-shared',
              'LIB_EXT'       => '.so',
              'OBJECT'        => 'SVM.o libsvm.o bindings.o',
              'XSOPT'         => '-C++',
              %args);
[download]

The only real difference I see, is that the "solaris" version of the file tells the system to create a shared library, where the default one does not.

The problem/resolution can reliably be reproduced by switching the build to use each of the differing makefiles.

One more note... I had to add #include <cstring> near the top of the bindings.cpp file to rid myself of undefined memcpy errors before the above worked properly.