In the absence of other information, be aware that the noble backslash escapes characters in Perl, though it serves as a path separator in the DOS and Windows worlds. In a double-quoted string, such as "C:\windmail\wm.exe", Perl will interpret the backslash as escaping the w characters.

One solution is to double up the backslashes. Another is to change them to forward slashes. Both will work, if this is truly the error you are seeing.