It should be noted that rrdtool comes with (optional) perl interface. If properly installed, you don't need external commands to access the data. For example (untested):

use RRDs;
...
my ($start,$step,$names,$data) = RRDs::fetch("$dir/$f",
        "AVERAGE", "--start", $begin, "--end", $end);
die if RRDs::error;

foreach (@$data) {
    print "@$_", "\n";
}
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The pipe and parsing are unnecessary and avoidable.