Ok, thanks.  So maybe like this?

#!/usr/bin/perl -lw
use strict;
use constant WINDOW => 30;

my $sum = 0;
my $n   = 0;
my $p   = WINDOW;

while (<DATA>) {
    my ($pos, $val) = split;
    if (eof) {  # corner case
        $sum += $val;
        $n++;
    }
    if ($pos > $p or eof) {
        print $sum / $n if $n > 0;
        $sum = 0;
        $n = 0;
        $p += WINDOW while $pos > $p;
    }
    $sum += $val;
    $n++;
}

__DATA__
1    1
10    3
30    1
40    2
60    2
[download]

Output:

1.66666666666667
2
[download]

(Upd: fixed handling of corner case)