Try running the command from the command line and displaying what windows sees as the exit code before perl gets it hands on it:

c:\test\xx>diff junk5.png junk6.png >nul & echo %errorlevel%
0

c:\test\xx>diff junk5.png junk4.png >nul & echo %errorlevel%
1
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The likely cause is that whilst Windows exit codes are ints, Perl internally truncates them to 8-bits before concatenating it with other bits that are meant to emulate child termination reasoning per the system docs:

If you'd like to manually inspect system's failure, you can check all 
+possible failure modes by inspecting $? like this:
    if ($? == -1) {
        print "failed to execute: $!\n";
    }
    elsif ($? & 127) {
        printf "child died with signal %d, %s coredump\n",
            ($? & 127),  ($? & 128) ? 'with' : 'without';
    }
    else {
        printf "child exited with value %d\n", $? >> 8;
    }
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