COMSPEC constantly reset to TCC.EXE

The context: I run a software that lauches Perl>CMD>make>CMD>CL at a very fast pace. If COMSPEC=<path>\TCC.EXE, TCC gets launched instead of CMD, but this regularly hangs (I would say launching many TCC processes at the same time triggers some problems with some ressources).
What I do: set COMSPEC=CMD.EXE. I also have added %COMSPEC in PROMPT in order to see the value of COMSPEC at any time.
The problem: COMSPEC is regularly reset to <path>\TCC.EXE, as I can see with the PROMPT and with...

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