I was able to reproduce the issue with this simple .xs: 
#include "EXTERN.h"
#include "perl.h"
#include "XSUB.h"

#include "ppport.h"


MODULE = Foo        PACKAGE = Foo        

void
test(class)
    char* class

    CODE:
        int ret = setenv("TESTME", "1", 0);
        warn("setenv TESTME complete with %d", ret);

[download]
compiling and running that like this: 
[karpet@freebsd80 ~/src/segfault-test/Foo]$ make test
PERL_DL_NONLAZY=1 /usr/bin/perl "-MExtUtils::Command::MM" "-e" "test_h
+arness(0, 'blib/lib', 'blib/arch')" t/*.t
t/Foo....ok 1/1setenv TESTME complete with 0 at t/Foo.t line 11.      
+       
t/Foo....dubious                                                      
+       
    Test returned status 0 (wstat 139, 0x8b)
    after all the subtests completed successfully
Failed Test Stat Wstat Total Fail  List of Failed
----------------------------------------------------------------------
+---------
t/Foo.t        0   139     1    0  ??
Failed 1/1 test scripts. 0/1 subtests failed.
Files=1, Tests=1,  0 wallclock secs ( 0.05 cusr +  0.08 csys =  0.12 C
+PU)
Failed 1/1 test programs. 0/1 subtests failed.
*** Error code 255

Stop in /usr/home/karpet/src/segfault-test/Foo.

[karpet@freebsd80 ~/src/segfault-test/Foo]$ gdb /usr/local/bin/perl pe
+rl5.8.9.core 
GNU gdb 6.1.1 [FreeBSD]
Copyright 2004 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and y
+ou are
welcome to change it and/or distribute copies of it under certain cond
+itions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB.  Type "show warranty" for det
+ails.
This GDB was configured as "i386-marcel-freebsd"...

warning: core file may not match specified executable file.
Core was generated by `perl5.8.9'.
Program terminated with signal 11, Segmentation fault.
Reading symbols from /usr/local/lib/perl5/5.8.9/mach/CORE/libperl.so..
+.done.
Loaded symbols for /usr/local/lib/perl5/5.8.9/mach/CORE/libperl.so
Reading symbols from /lib/libm.so.5...done.
Loaded symbols for /lib/libm.so.5
Reading symbols from /lib/libcrypt.so.5...done.
Loaded symbols for /lib/libcrypt.so.5
Reading symbols from /lib/libutil.so.8...done.
Loaded symbols for /lib/libutil.so.8
Reading symbols from /lib/libc.so.7...done.
Loaded symbols for /lib/libc.so.7
Reading symbols from /usr/home/karpet/src/segfault-test/Foo/blib/arch/
+auto/Foo/Foo.so...done.
Loaded symbols for /usr/home/karpet/src/segfault-test/Foo/blib/arch/au
+to/Foo/Foo.so
Reading symbols from /libexec/ld-elf.so.1...done.
Loaded symbols for /libexec/ld-elf.so.1
#0  0x282b85fc in free () from /lib/libc.so.7
(gdb) bt
#0  0x282b85fc in free () from /lib/libc.so.7
#1  0x2832b5eb in strnlen () from /lib/libc.so.7
#2  0x28265b00 in ?? () from /lib/libc.so.7
#3  0x28343000 in ?? () from /lib/libc.so.7
#4  0x2807b358 in ?? () from /libexec/ld-elf.so.1
#5  0xbfbfe9f8 in ?? ()
#6  0x283355ac in ?? () from /lib/libc.so.7
#7  0x2808d100 in ?? ()
#8  0x2807b358 in ?? () from /libexec/ld-elf.so.1
#9  0xbfbfe9f8 in ?? ()
#10 0x2804ed25 in dlsym () from /libexec/ld-elf.so.1
Previous frame inner to this frame (corrupt stack?)
(gdb)

[download]
that stack trace is identical to the one I'm seeing in my own module.

In reply to Re^2: Test::More wait status non-zero but all tests pass by karpet
in thread Test::More wait status non-zero but all tests pass by karpet

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