sub main {

my $start = CORE::time();
# do your stuff here
my $end = CORE::time();

my $time_taken = $end - $start; # gives us the number of seconds it took to run...

my $secs_to_sleep = (5 * 60) - $time_taken; # 5 mins worth of seconds, minus the time already taken

# then do a "sleep" to make it wait until that time has come...
sleep $secs_to_sleep;
}

#!/usr/bin/perl

$logfile = "your_log_file.log";

open $log, '<', $logfile;
my $position;

while (1) {
while (my $line = <$log>) {

# Skip the line if it's being writen atm
last unless substr($line, -1) eq "\n";

# Do some stuff here

# When finished, save the postion for later
$position = tell $log;
}

# Sleep
sleep (300); # Sleep for 5 min

# Skip the lines we already parsed
seek $log, $position, 0;
}