use strict;
use warnings;
sub mean {...}
my @genes = (
#'GeneName E1  E2  E3  E4',
'ATA1      12  44  45  33',
'OSA2     100  79  85  83',
'DUA5      66  65  64  67',
'AXANT      4   4   6   2',
);
foreach  (@genes) {
    my ($name, @values) = split;
    print  $name, mean(@values);
}
[download]

It needs an implementation of the 'mean' function. It uses a hard coded array of strings rather than input. The output is unformatted. The default arguments of 'split' may not work as intended. Clearly, the highest priority is the function. I am not ashamed to use a module to do the heavy work.

use List::Util qw(sum);
...
sub mean { return sum(@_)/@_ }
[download]

Now you should have a working program. Add input and output details until it meets your requirements. As a final step, you may want to replace the module with your own sum function.