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';
$ 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,
looks for the following REG_SZ/REG_EXPAND_SZ registry variables, and i
either or both are present, they are executed first.