Some more hacking around gives me this. The important change is no more dependency on errors.err. Instead, we use vim's own shell-out buffers, which are better. Plus, the old perl -wc wasn't working for me. And, this is also faster than using system() and gives immediate results, which is nice when you're waiting for 20+ testfiles to run. :-)

function! Prove ( verbose, taint )
    if ! exists("g:testfile")
        let g:testfile = "t/*.t"
    endif
    if g:testfile == "t/*.t" || g:testfile =~ "\.t$"
        let s:params = "l"
        if a:verbose
            let s:params = s:params . "v"
        endif
        if a:taint
            let s:params = s:params . "Tt"
        endif
        execute "!prove -" . s:params . " " . g:testfile
    else
       call Compile ()
    endif
endfunction

function! Compile ()
    if ! exists("g:compilefile")
        let g:compilefile = expand("%")
    endif
    execute "!perl -wc -Ilib " . g:compilefile
endfunction

nmap ,t :call  Prove (0,0)<cr>
nmap ,tt :call  Prove (0,1)<cr>
nmap ,T :call  Prove (1,0)<cr>
nmap ,TT :call  Prove (1,1)<cr>
nmap ,v :call  Compile ()<cr>
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