If you interested in the operator interface more than the details of the algorithm, use of perl modules can simplify the progam and provide improved error reporting/recovery.

use strict;
use warnings;
use IO::Prompt::Hooked;
use List::Util qw(max min sum);
my %prompt_options = (
    message  => 'input a positive integer (or a negative integer to st
+op)',
    validate => qr/^-?\d+$/,
    error    => "Input must be a positive integer\n",
    escape   => qr/^-\d/,
); 

my @input_array;
push @input_array, $_ while (defined ($_ = prompt(%prompt_options)));
die "No inputs were entered\n" if (!@input_array) ;

print "\n\n\n";
print 'Highest input: ', max( @input_array ), "\n";
print 'Lowest input:  ', min( @input_array ), "\n";
print 'average:       ', sum( @input_array ) / @input_array, "\n";
[download]

This approach is wasteful of both computer time and memory, but neither should be a problem with the small amount of data that a human operator is likely to enter. The advantage is that this is easy to write and you can hve confidence in the computations done by the modules.