Here's a complete working example:

package Foo;
use strict;
use warnings;
use Errno "EINVAL";
use Tie::Scalar;
use AutoLoader;
use Carp;

sub constant { $!=EINVAL() }
our $AUTOLOAD;
sub AUTOLOAD {
    # this is used to 'autoload' constants from the constant()
    # XS function.  If a constant is not found then control is passed
    # to the AUTOLOAD in AutoLoader.

    my $constname;
    ($constname = $AUTOLOAD) =~ s/.*:://;
    croak "& not defined" if $constname eq 'constant';
    my $val = constant($constname, @_ ? $_[0] : 0);
    
    if ($! != 0) {
        if ($! =~ /Invalid/ || $!{EINVAL}) {
            $AutoLoader::AUTOLOAD = $AUTOLOAD;
            goto &AutoLoader::AUTOLOAD;
        } else {
            croak "Your vendor has not defined MMA macro $constname";
    }   }
    eval "sub $AUTOLOAD { $val }";
    goto &$AUTOLOAD;
}
1;
__END__

sub TIESCALAR { goto &Tie::StdScalar::TIESCALAR }
sub FETCH { goto &Tie::StdScalar::FETCH }
sub STORE { goto &Tie::StdScalar::STORE }
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$ perl -wle'use Foo; tie $x, "Foo"; $x="42"; print $x; print for grep 
+/auto/, values %INC'
42
auto/Foo/autosplit.ix
./auto/Foo/STORE.al
./auto/Foo/TIESCALAR.al
./auto/Foo/FETCH.al
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I put in a

use Errno qw(EINVAL);
use Tie::Scalar;
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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;
}
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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.
$
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To complete the picture, here's line 42 of the .t file:

ok (tie(my $tiedScalar, 'IPC::MMA::Scalar', $scalar), "tie scalar");
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Can you (anyone) think of some other requirement I've overlooked, or where else things might have gone wrong?

It occurs that I have a problem with naming structure inherited from the guy who wrote the package that I'm working to supercede. He (I) have a module/package named IPC::MM (IPC::MMA).

Using this, people want to tie to names IPC::MM::Scalar and IPC::MM::Hash (IPC::MMA::Scalar, IPC::MMA::Array, IPC::MMA::Hash). The way he did this was to write out the names of the tie routines in full in MM.pm, like this:

# Preloaded methods go here.
sub IPC::MM::Scalar::TIESCALAR {
    my $class = shift;
    my ($val) = @_;
    return bless \$val, $class;
}
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This works in preloaded form, but it seems it can't be autoloaded.

In my .xs file there is a nice little header:

MODULE = IPC::MMA       PACKAGE = IPC::MMA
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and one can have other headers that vary these names. But I don't know of any comparable structure on the .pm side. Assuming that I want to keep the inherited naming structure, what can I do in the .pm file (files?) to straighten out this mess?

cmac
www.animalhead.com

You shouldn't need Tie::Scalar; I just had that because my tie routines used Tie::StdScalar stuff from it.
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