use strict;
use warnings;

my %data;

for my $file ('file1.txt', 'file2.txt', 'file3.txt',
           'file4.txt' , 'file5.txt') {
    open(my $handle, '<', $file) or die "Open fail on $file: $!\n";
    while (<$handle>) {
        chomp;
        my ($key, $val) = split;
        $data{$key}{'count'}++;
        $data{$key}{'sum'} += $val;
    }
}

open(my $handle, '>', 'average.txt') or die "Open fail on average.txt: $!\n";
for my $key (sort { $a <=> $b } keys %data) {
    $data{$key} = $data{$key}{'sum'} / $data{$key}{'count'};
    print $handle "$key\t$data{$key}\n";
}