use strict;
use warnings;

my ($file, $handle, $key, $val, %data);

for $file ('file1.txt', 'file2.txt', 'file3.txt',
           'file4.txt' , 'file5.txt') {
    open($handle, $file);
    while (<$handle>) {
        chomp;
        ($key, $val) = split /\s/, $_;
        $data{$key}{'count'}++;
        $data{$key}{'sum'} += $val;
    }
}

open($handle, '>average.txt');
for $key (sort { $a <=> $b } keys %data) {
    $data{$key} = $data{$key}{'sum'} / $data{$key}{'count'};
    print $handle "$key\t$data{$key}\n";
}
close($handle);