#/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
}