$!      If used in a numeric  context,  yields  the  current
             value  of  errno, with all the usual caveats.  (This
             means that you shouldn't depend on the value  of  $!
             to  be anything in particular unless you've gotten a
             specific error return indicating  a  system  error.)
             If  used in a string context, yields the correspond-
             ing system error string.  You can assign  to  $!  in
             order  to set errno if, for instance, you want $! to
             return the string for error n, or you  want  to  set
             the  exit  value  for  the die operator.  (Mnemonic:
             What just went bang?)