Only a couple small points. The corrected code follows

The .= operator appends anything on the righthand side to the left.. So you use either x = x . y; or x .= y; but not x .= x . y;

In order for the calculated sum to be available outside of the foreach loop, it must be declared outside of the foreach loop. This also means that it can't be redeclared with "my" inside the foreach loop or in the ensuing print statement.

use strict;
use warnings;
my $i = 0;
my $input = "";
while ($i < 3) {
        chomp(my $num = <>);
        $input .= " $num";
        $i++;
}
print "Total input is $input\n";
my @nums = split(' ', $input);

my $sum;
foreach (@nums) {
        $sum += $_;
}
print $sum / @nums . "\n";
exit;
[download]

There are many other ways this code can be improved. Hopefully the above points get you going again though.

Hello stamp1982,

You should provide a prompt to alert the user that input is required. For example: print "Enter a list of sequence lengths, separated by whitespace:\n";. And this line: $input .= $num . "". $num; is wrong; it should be something like $input .= $num . " ";. Also, to split on whitespace, you should use split(/\s+/, $input);. See split.

But the main problem arises from your repeated declarations of the $sum variable. my is used to declare and initialise a lexical variable (and if no initialisation is given explicitly, a scalar variable is by default initialised to undef). So the final few lines should be:

print "Total input is: $input\n";

my @nums = split(/\s+/, $input);

my $sum;
   $sum += $_ for @nums;

print $sum / @nums . "\n";
[download]

Note that $sum is here declared (and initialised) once only. Any further use of my on $sum would create a new variable with the same name, which is not what you want. See e.g. Variable scoping.

Hope that helps,