More exact timing can be achieved by properly calculating the interval. This is a rough sketch, taken from memory

#/usr/bin/env perl
use strict;
use warnings;

# We need sub-second precision here :-)
use Time::HiRes wq(time sleep);

my $lastrun = 0;
while(1) {
    # Time consuming stuff here

     my $now = time;
     # Calculate the time we need to sleep. First we calculate the dif
+ference between now and the last run.
     # That's how long the last run took. Now, calculate how many seco
+nds remaining in the current minute.
     # If the answer is negative, one of two things happened: Either i
+t's our first run, or the last run took
     # longer than a minute
     my $sleeptime = 60 - ($now - $lastrun);
     if($sleeptime > 0) {
         sleep($sleeptime);
     }
     $lastrun = $now;
}
[download]

If you want the code to run at a specific second in every minute, you could also do that. Again, this is untested and from memory:

#/usr/bin/env perl
use strict;
use warnings;

# We need sub-second precision here :-)
use Time::HiRes wq(time sleep);

my $activesecond = 42; # Run whenever the seconds are "42"

while(1) {
    my $now = time;

    # The built-in modulus function converts to integer,
    # which would introduce jitter of up to nearly a second.
    # So, out with the traditional:
    # my $cursecond = $now % 60;
    # ...and in with the more manual version:
    my $cursecond = $now - (60.0 * int($now / 60.0));

    if($cursecond != $activesecond) {
        # Need to wait
        my $sleeptime = $activesecond - $cursecond;
        if($sleeptime < 0) {
            # Handle rollover
            $sleeptime += 60.0;
        }
        sleep($sleeptime);
    }

    # Time consuming stuff here
}
[download]

Hope that helps a bit.

Edit: If, for some strange reason you need to use International Atomic Time, you need to take the UTC/TAI offset into account. This is currently 37 seconds, but you will need to consult the IERS Bulletin C twice a year to check for leap second announcements. But in essence, all you need to do is add the appropriate offset when assigning $now:

my $taioffset = 37;

...

    my $now = $time + $taioffset;
[download]

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