I put in a

use Errno qw(EINVAL);
use Tie::Scalar;
[download]

as you had, and a couple of print statements in my AUTOLOAD:

sub AUTOLOAD {
    my $constname;
    ($constname = $AUTOLOAD) =~ s/.*:://;
    croak "& not defined" if $constname eq 'constant';
    my $val = constant($constname, @_ ? $_[0] : 0);
    
    if ($! != 0) {
        if ($! =~ /Invalid/ || $!{EINVAL}) {
            print "just before AUTOLOAD\n";
            $AutoLoader::AUTOLOAD = $AUTOLOAD;
            goto &AutoLoader::AUTOLOAD;
        } else {
            croak "Your vendor has not defined MMA macro $constname";
    }   }
    print "just before eval\n";
    eval "sub $AUTOLOAD { $val }";
    goto &$AUTOLOAD;
}
[download]

No sign of output from the print statements. Here is the output, both under test harness and standalone:

$ make test
PERL_DL_NONLAZY=1 /usr/bin/perl "-MExtUtils::Command::MM" "-e" "test_h
+arness(0, 'blib/lib', 'blib/arch')" t/*.t
t/6_tiedScalars....1/18 Undefined subroutine &IPC::MMA::Scalar::TIESCA
+LAR called at t/6_tiedScalars.t line 42.
# Looks like you planned 18 tests but ran 4.
# Looks like your test exited with 255 just after 4.
t/6_tiedScalars.... Dubious, test returned 255 (wstat 65280, 0xff00)
 Failed 14/18 subtests 

Test Summary Report
-------------------
t/6_tiedScalars (Wstat: 65280 Tests: 4 Failed: 0)
  Non-zero exit status: 255
  Parse errors: Bad plan.  You planned 18 tests but ran 4.
Files=1, Tests=4,  0 wallclock secs ( 0.03 usr  0.00 sys +  0.05 cusr 
+ 0.00 csys =  0.09 CPU)
Result: FAIL
Failed 1/1 test programs. 0/4 subtests failed.
*** Error code 255

Stop in /build/IPC-MMA-0.5.
$ t/6_tiedScalars.t
1..18
ok 1 - created shared mem
ok 2 - read available mem
ok 3 - make scalar
ok 4 - effect on available mem is -16
Undefined subroutine &IPC::MMA::Scalar::TIESCALAR called at t/6_tiedSc
+alars.t line 42.
# Looks like you planned 18 tests but ran 4.
# Looks like your test exited with 255 just after 4.
$
[download]

To complete the picture, here's line 42 of the .t file:

ok (tie(my $tiedScalar, 'IPC::MMA::Scalar', $scalar), "tie scalar");
[download]

Can you (anyone) think of some other requirement I've overlooked, or where else things might have gone wrong?