Many thanks for your continued help.

Running with the -xs flag I still get the same result during tests:

t/weak..........skipped
        all skipped: weaken requires XS version
[download]

I looked at the code for Module::Install:Can, and the guts involve $Config::Config{cc} so I did check:

$ perl -MConfig -e 'print $Config::Config{cc}'
gcc
[download]

Which I guess isn't surprising since I haven't had troubles building other xs-based modules. The check in weak.t looks at @Scalar::Util::EXPORT_FAIL, and it does look like the code being executed is in: perl5.8.7/lib/5.8.7/Scalar/Util.pm.

So after that I'm still not quite sure what's going on. In the make output I see:

gcc -c   -D_ALL_SOURCE -D_ANSI_C_SOURCE -D_POSIX_SOURCE -DUSE_NATIVE_D
+LOPEN -fno-strict-aliasing -pipe -I/usr/local/include -D_LARGE_FILES 
+-O   -DVERSION=\"1.18\" -DXS_VERSION=\"1.18\"  "-I/db2blast/Paul/perl
+5.8.7/lib/5.8.7/aix/CORE"  -DPERL_EXT Util.c
Running Mkbootstrap for List::Util ()
chmod 644 Util.bs
rm -f blib/arch/auto/List/Util/Util.so
gcc  -Wl,-bhalt:4 -Wl,-bexpall -Wl,-G -Wl,-bnoentry -lc -L/usr/local/l
+ib Util.o  -o blib/arch/auto/List/Util/Util.so    \
        \

chmod 755 blib/arch/auto/List/Util/Util.so
cp Util.bs blib/arch/auto/List/Util/Util.bs
chmod 644 blib/arch/auto/List/Util/Util.bs
[download]

and during make install I get:

Installing /db2blast/Paul/perl5.8.7/lib/5.8.7/aix/auto/List/Util/Util.
+so
Files found in blib/arch: installing files in blib/lib into architectu
+re dependent library tree
Writing /db2blast/Paul/perl5.8.7/lib/5.8.7/aix/auto/List/Util/.packlis
+t
Appending installation info to /db2blast/Paul/perl5.8.7/lib/5.8.7/aix/
+perllocal.pod
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Which I had thought indicates successful building of the xs code. I also tried uninstalling with:

cpan> o conf make_install_arg UNINST=1
    make_install_arg   [UNINST=1]

cpan> force install Scalar::Util
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Which led to the same result. Might you have any other ideas of what I could try or where I could look?