Adding the /useenv means that one's PATH actually gets through. This is

Trent Mick · Trent Mick · commit b1386772b586 · 2006-03-12T23:40:58.000Z
important for the _ssl.vproj build. It calls build_ssl.py which tries to
find a Perl to use. Without "/useenv" Visual Studio is getting a PATH
from somewhere else (presumably from its internal environment
configuration). The result is that build_ssl.py fallsback to its
"well-known" locations for a Perl install.
diff --git a/Tools/buildbot/build.bat b/Tools/buildbot/build.bat
@@ -1,3 +1,3 @@
 @rem Used by the buildbot "compile" step.
 call "%VS71COMNTOOLS%vsvars32.bat"
-devenv.com /build Debug PCbuild\pcbuild.sln
+devenv.com /useenv /build Debug PCbuild\pcbuild.sln
