The undefined references should be resolved by linking to the pthreads library.
Strawberry seems to provide pthreads - there's a c\x86_64-w64-mingw32\lib\libpthread.a which is probably the import library to which you should link.

I think the corrected version of the failing g++ -o ... command would have: 
 -L"C:\strawberry\c\x86_64-w64-mingw32\lib -lpthread"

[download]
inserted between the -lcomtl32 and the dll.exp.
You could 'cd' to the build directory and run by hand (copy'n'paste it in to the command line, then hit 'Enter') that *corrected* version of that failing command.
Then run dmake test and dmake install
Or you could find some other way of correcting the build procedure.

The strawberry mailing list could probably provide good help if all else fails.

Cheers,
Rob

UPDATE: I just tried the hack I suggested above, and it worked fine for me. (Note that my libpthread.a is in a different location to the OP's. Modify accordingly.) 

C:\_64\strawberry518>cpan -i Imager::QRCode
.
.
.
g++.exe -o blib\arch\auto\Imager\QRCode\QRCode.dll -Wl,--base-file -Wl
+,dll.base -mdll -s -L"C:\_64\strawberry518\perl\lib\CORE" -L"C:\_64\s
+trawberry518\c\lib" src/bitstream.o  src/mask.o      src/mmask.o     
+src/mqrspec.o   src/QRCode.o src/qrencode.o  src/qrinput.o   src/qrsp
+ec.o    src/rscode.o    src/split.o   C:\_64\strawberry518\perl\lib\C
+ORE\libperl518.a -lmoldname -lkernel32 -luser32 -lgdi32 -lwinspool -l
+comdlg32 -ladvapi32 -lshell32 -lole32 -loleaut32 -lnetapi32 -luuid -l
+ws2_32 -lmpr -lwinmm -lversion -lodbc32 -lodbccp32 -lcomctl32 dll.exp
src/mqrspec.o:mqrspec.c:(.text+0x188): undefined reference to `pthread
+_mutex_lock'
src/mqrspec.o:mqrspec.c:(.text+0x19b): undefined reference to `pthread
+_mutex_unlock'
src/mqrspec.o:mqrspec.c:(.text+0x31c): undefined reference to `pthread
+_mutex_lock'
src/mqrspec.o:mqrspec.c:(.text+0x346): undefined reference to `pthread
+_mutex_unlock'
c:/_64/strawberry518/c/bin/../lib/gcc/x86_64-w64-mingw32/4.6.3/../../.
+./../x86_64-w64-mingw32/bin/ld.exe: src/mqrspec.o: bad reloc address 
+0x0 in section `.pdata'
c:/_64/strawberry518/c/bin/../lib/gcc/x86_64-w64-mingw32/4.6.3/../../.
+./../x86_64-w64-mingw32/bin/ld.exe: final link failed: Invalid operat
+ion
collect2: ld returned 1 exit status
dmake.EXE:  Error code 129, while making 'blib\arch\auto\Imager\QRCode
+\QRCode.dll'
  KURIHARA/Imager-QRCode-0.034.tar.gz
  C:\_32\strawberry514\c\bin\dmake.EXE -- NOT OK
Running make test
  Can't test without successful make
Running make install
  Make had returned bad status, install seems impossible

C:\_64\strawberry518>cd \Users\sisyphus\.cpan\build\Imager-QRCode-0.03
+4-agp6k1

C:\Users\sisyphus\.cpan\build\Imager-QRCode-0.034-AGp6k1>g++.exe -o bl
+ib\arch\auto\Imager\QRCode\QRCode.dll -Wl,--base-file -Wl,dll.base -m
+dll -s -L"C:\_64\strawberry518\perl\lib\CORE" -L"C:\_64\strawberry518
+\c\lib" src/bitstream.o  src/mask.o      src/mmask.o     src/mqrspec.
+o   src/QRCode.o src/qrencode.o  src/qrinput.o   src/qrspec.o    src/
+rscode.o    src/split.o   C:\_64\strawberry518\perl\lib\CORE\libperl5
+18.a -lmoldname -lkernel32 -luser32 -lgdi32 -lwinspool -lcomdlg32 -la
+dvapi32 -lshell32 -lole32 -loleaut32 -lnetapi32 -luuid -lws2_32 -lmpr
+ -lwinmm -lversion -lodbc32 -lodbccp32 -lcomctl32 -L"C:\_64\strawberr
+y518\c\x86_64-w64-mingw32\lib" -lpthread dll.exp

C:\Users\sisyphus\.cpan\build\Imager-QRCode-0.034-AGp6k1>dmake
C:\_64\strawberry518\perl\bin\perl.exe "-Iinc" -MExtUtils::Command -e 
+cp -- QRCode.bs blib\arch\auto\Imager\QRCode\QRCode.bs
C:\_64\strawberry518\perl\bin\perl.exe "-Iinc" -MExtUtils::Command -e 
+chmod -- 644 blib\arch\auto\Imager\QRCode\QRCode.bs

C:\Users\sisyphus\.cpan\build\Imager-QRCode-0.034-AGp6k1>dmake test
C:\_64\strawberry518\perl\bin\perl.exe "-MExtUtils::Command::MM" "-e" 
+"test_harness(0, 'inc', 'blib\lib', 'blib\arch')" t/00.load.t t/01.pl
+ot.t
t/00.load.t .. # Testing Imager::QRCode 0.034
t/00.load.t .. ok
t/01.plot.t .. ok
All tests successful.
Files=2, Tests=5,  0 wallclock secs ( 0.05 usr +  0.03 sys =  0.08 CPU
+)
Result: PASS

C:\Users\sisyphus\.cpan\build\Imager-QRCode-0.034-AGp6k1>

[download]

In reply to Re: Installing Imager::QRCode on windows 8 by syphilis
in thread Installing Imager::QRCode on windows 8 by shohn

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