Perl5 リファレンス

       pos SCALAR

       pos     Returns the offset of where the last `m//g' search
               left off for the variable is in question (`$_' is
               used when the variable is not specified).  May be
               modified to change that offset.  Such modification
               will also influence the `\G' zero-width assertion
               in regular expressions.  See the perlre manpage
               and the perlop manpage.