$start = "23:58:45";
$end   = "00:01:15";
$key = $start;
while ($key ne $end) {
   print "$key\n"; # perform action on $hash{$key} here
   ($h, $m, $s) = split(":",$key);
   (($s +=1) %=60) || (($m +=1) %=60) || (($h +=1) %=24); 
   $key = join(":",map{sprintf("%02d",$_)}($h,$m,$s));
}