#!/usr/bin/perl -w
use strict;

my %do_op=(
  add       => sub{$_[0] +  $_[1]},
  multiply  => sub{$_[0] *  $_[1]},
  divide    => sub{$_[0] /  $_[1]},
  subtract  => sub{$_[0] -  $_[1]},
  power	   => sub{$_[0] ** $_[1]}, 
  equal  	=> sub{$_[0] == $_[1]? $_[0]:0},
  max    	=> sub{$_[0] >  $_[1]? $_[0]:$_[1]}, 
  min    	=> sub{$_[0] <  $_[1]? $_[0]:$_[1]},        
);

sub operate{ 
        my $op=shift;
        my $tot = shift;           
        while (defined (my $x=shift)){
 				  $tot = $do_op{$op}->($tot,$x);
		  }
		  $tot;
};   		 

my $op = shift or die "You must specify an operation followed by numbers.\n";
die "Non-numeric input.\n" if grep /\D/, @ARGV;
die "Invalid operation.\n" unless exists $do_op{$op};

print "$op (@ARGV)= ". operate($op, @ARGV) . "\n";