CPU_start(); CPU_max(30) while 1;

##</code><code>##

use Time::HiRes qw(time);

sub CPU_used {
    (map {$_->[13]+$_->[14]} 
        [split " ", Cat("/proc/self/stat")])[0]
}

{ my %start = (tm => 0, cpu => 0);
sub CPU_start { @start{"tm","cpu"} = (time(),CPU_used()) }
sub CPU_max {
    my ($max, $real, $cpu) = ($_[0], time()-$start{tm},
        CPU_used()-$start{cpu});
    return unless defined($max) and $max > 0;
    &sleep( $cpu/$max-$real );
}}

#
# macro used from CPU_used() and CPU_max()
#

sub sleep { select undef,undef,undef,$_[0] }
sub Cat {
	local *F;
	open F, "< ".$_[0] or return;
	local $/ unless wantarray;
	return <F>;
}