After you are done with collecting values, even after "$total--" , your $total is one higher than it should be.

This is because you increment $total before putting anything into the corresponding index of @value.

The simplistic fix would be to do an additional $total--; But you will need to divide by $total+1 (Number of elements).

I think the main issue is that you expect $total to be both an index, and a count of number of values.

There are numerous other coding style suggestions - I'm sure others will chime in.

Update: Here is a simplistic, more perlish rewrite of your code, including some error checking.

#!/usr/bin/perl
use strict;
use warnings;

print "\nMean Average Program\n\nenter values, when done enter 'done':
+";

my @value;

while (my $input=<>){
  chomp $input;
  last if uc($input) eq "DONE";
  #Validate that the input is only digits-should reallly [use Scalar::
+Util qw(looks_like_number)]
  print ("INVALID value '$input' -ignored\nTry again:") , next
        unless $input =~ /^[+-]?\d+(\.\d+)?$/; # Crude floating-point 
+test
  push @value, $input ;
  print "Enter a value('done' if finished): ";
}

my $mean = 0;
for my $val (@value){
  $mean += $val;
}
if (scalar @value){ # There is at least one @value - avoid div by 0
   $mean = $mean / scalar(@value);
}

print "\nThe mean of your values is $mean\n";
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