...
my $targettimetext = "13:14:15";

# Turn target time into seconds
my @tttparts = split/\:/, $targettimetext;
my $targettime = ($tttparts[0] * 3600) + ($tttparts[1] * 60) + $tttparts[2];
...

while(1) {
    # Turn the current time into seconds
    my ($sec,$min, $hour, $mday,$mon, $year, $wday,$yday, $isdst) = localtime time;
    my $nowtime = ($hour * 3600) + ($minute * 60) + $sec;
    
    # Not yet time to do things? Sleep until it is
    if($nowtime < $targettime) {
        sleep($targettime - $nowtime);
    }

    # YOUR STUFF HERE
    
    # Recalculate the current time after doing stuff
    ($sec,$min, $hour, $mday,$mon, $year, $wday,$yday, $isdst) = localtime time;
    $nowtime = ($hour * 3600) + ($minute * 60) + $sec;
    
    # Sleep for the rest of the day
    sleep(86400 - $nowtime);
}