Fix checks for tainted dir in $ENV{PATH}

Fix checks for tainted dir in $ENV{PATH}

$ cat > foo
#!/usr/bin/perl
print "What?!\n"
^D
$ chmod +x foo
$ ./perl -Ilib -Te '$ENV{PATH}="."; exec "foo"'
Insecure directory in $ENV{PATH} while running with -T switch at -e line 1.

That is what I expect to see.  But:

$ ./perl -Ilib -Te '$ENV{PATH}="/\\:."; exec "foo"'
What?!

Perl is allowing the \ to escape the :, but the \ is not treated as an
escape by the system, allowing a relative path in PATH to be consid-
ered safe.