Calculating CPU Usage from /proc/stat in Perl

cpuusage.pl

#!/usr/bin/perl

# Ref: Calculating CPU Usage from /proc/stat
# (http://colby.id.au/node/39)

use strict;
use warnings 'all';
use utf8;

use List::Util qw(sum);

$| = 1;

my ($prev_idle, $prev_total) = qw(0 0);
while () {
        open(STAT, '/proc/stat') or die "WTF: $!";
        while (<STAT>) {
                next unless /^cpu\s+[0-9]+/;
                my @cpu = split /\s+/, $_;
                shift @cpu;

                my $idle = $cpu[3];
                my $total = sum(@cpu);

                my $diff_idle = $idle - $prev_idle;
                my $diff_total = $total - $prev_total;
                my $diff_usage = 100 * ($diff_total - $diff_idle) / $diff_total;

                $prev_idle = $idle;
                $prev_total = $total;

                printf "CPU: %0.2f%%  \r", $diff_usage;
        }
        close STAT;

        sleep 1;
}