Yeah, this is due to a known problem with whitespace following the '}' close brace on the line preceding the 'else'. We have a CR for this and will get it fixed in the next release. Getting this extra whitespace is a really easy thing to do if you are at all cutting and pasting.
In the meantime, either be extra sure there is no trailing whitespace at the end of your lines or simply put the 'else {' on the same line as in:
when HTTP_REQUEST {
if { [matchclass [HTTP::host] equals $::CorpWebHostNames] } {
use pool CorpWeb
} else {
use pool ProdWeb
}
}