cat some_file_holding3numbers.txt |  avg_routines.pl (+some way of specifying avg3())> avg.txt

##</code><code>##

perl -w avg_routines.pl (+some way of specifying avg3()) <some_file_holding3numbers.txt >avg.txt

##</code><code>##

  my $Routine = shift;  #  Get routine name from cmd line
  &{ $Routine };        #  Call named routine