function! ShortenURL()
  " This function grabs the url under the cursor, passes it to an 
  " external script, grabs the shorter, returned url and replaces any
  " occurrences of the original URL. It then resets the cursor to the
  " starting posistion or the end of line if the column is now empty.

  " get the starting posistion
  let s:line  = line(".")
  let s:colno = col(".")

  " this matches URLs!
  let s:lurl = expand('<cWORD>')

  "shorten the URL. this invokes the external command.

  let s:surl = system("masl.pl -n '" . s:lurl . "'")

  "now we have the long and short of it, replace long with short.
  execute "1,$s!" . s:lurl . "!" . s:surl . "!"

  " return us to where the command was invoked from. Will need later
  execute "call cursor(s:line, s:colno)"
endfunction

" This is the key that you use to call the function.
noremap <f3> <Esc>:call ShortenURL()<CR>