use Date::Calc qw(Delta_DHMS);
$start_time = "16:54:23";
$end_time = "16:54:28";
@random=(2001,1,1);  #Any value to allay the ymd filler
push @end, @random, split/:/, $end_time;
push @start, @random, split/:/, $start_time;
@diff=Delta_DHMS(@start,@end);
print join":",@diff[1..3];