use strict;
use warnings;

sub hms_to_s {
    my ($hms) = @_;
    my ($h,$m,$s) = split /:/, $hms;
    return $h*60*60 + $m*60 + ($s||0);
}

for (
    [qw( 08:00:00 17:00:00 )],
    [qw( 23:00:00 06:00:00 )],
) {
    my ($time1_hms, $time2_hms) = @$_;

    my $time1 = hms_to_s($time1_hms);
    my $time2 = hms_to_s($time2_hms);

    my $diff = ( ( $time2 + 24*60*60 ) - $time1 ) % (24*60*60);

    my $diff_h = int( $diff / (60*60) );
    my $diff_m = int( $diff / 60      ) % 60;
    my $diff_s =      $diff             % 60;

    printf("%s -> %s: %02d:%02d:%02d\n",
        $time1_hms,
        $time2_hms,
        $diff_h,
        $diff_m,
        $diff_s,
    );
}

##</code><code>##

08:00:00 -> 17:00:00: 09:00:00
23:00:00 -> 06:00:00: 07:00:00