Avoid mixing Strawberry perl with your own mingw installation unless you exactly know what can happen. I recommend to remove all the other mingws from your PATH when building Win32::GUI in strawberry perl.

Also assure that you do not have some other copy of dmake.exe in your PATH before strawberry's directories (IIRC Win32::GUI generate slightly strange Makefile which can be refused by the original dmake - e.g. https://rt.cpan.org/Public/Bug/Display.html?id=53721)

Win32::GUI should build fine with Strawberry perl 5.10.1 with mingw toolchain included with strawberry perl.

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kmx