If this variable is set to a true value, then stat() on Windows will not try to open the file. This means that the link count cannot be determined and file attributes may be out of date if additional hardlinks to the file exist. On the other hand, not opening the file is considerably faster, especially for files on network drives.
This variable could be set in the sitecustomize.pl file to configure the local Perl installation to use "sloppy" stat() by default. See the documentation for -f in perlrun for more information about site customization.