I like toolic's solution, but if you prefer not to use a module:

use strict;
use warnings;
my $sum;
my $count;
while (<DATA>) {
    my $length = (split)[3];
    next if $length eq 'length';
    $count++;
    $sum += $length;
}
my $average_length = $sum/$count;
print $average_length, "\n";
__DATA__
chr    start    end    length
chr1   10       50      40
chr2    20      80      60
[download]