1. $_ is also called $ARG if you 'use English;', what does the three letters 'ARG' mean? It means there are certain functions/operations, if you omit the arg, Perl will take $ARG as the default arg. In perlvar/$_, there is a complete list on this.
   
   
2. $_ is also the place to store values, under specified situations. One example is, as stated in perlvar/$_:
   
   "The default place to put an input record when a <FH> operation's result is tested by itself as the sole criterion of a while test. Outside a while test, this will not happen."

Simple, that's it. Document, and again document. Perl does deliver what perlvar promised, but not those it never promised.