Ok, the problem was autoruns

reg query "HKCU\Software\Microsoft\Command Processor" /s

How to change the default startup directory for Command Prompt?

Hidden gotcha: The command processor's AutoRun setting - The Old New Thing - Site Home - MSDN Blogs

This could (or not) be considered a win32 perl bug as SH_PATH/Config#sh is lacking a /D option

$ perl -V:sh
sh='cmd /x /c';
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$ cmd /?
...
/D      Disable execution of AutoRun commands from registry (see below
+)

...
If /D was NOT specified on the command line, then when CMD.EXE starts,
+ it
looks for the following REG_SZ/REG_EXPAND_SZ registry variables, and i
+f
either or both are present, they are executed first.

    HKEY_LOCAL_MACHINE\Software\Microsoft\Command Processor\AutoRun

        and/or

    HKEY_CURRENT_USER\Software\Microsoft\Command Processor\AutoRun
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