See "Path-style conflict" in "MDEV-30836 MTR Cygwin fix" for explanation.

To install subshell fix use --cygwin-subshell-fix=do
To uninstall use --cygwin-subshell-fix=remove

This works only from Cygwin environment. As long as perl on PATH is
from Cygwin you are on Cygwin environment. Check it with

     perl --version

     This is perl 5, version 36, subversion 1 (v5.36.1) built for
     x86_64-cygwin-threads-multi