while (<DATA>){
    my $former_hour = substr($_, 0, 2);
    my $former_mn = substr($_, 2, 2);

    my $new_hour = ($former_hour + int($diff / 60)) % 24;
    my $new_mn = ($former_mn + $diff) % 60;

    print STDOUT "former_time is $former_hour$former_mn\n";
    print STDOUT "new time is $new_hour$new_mn\n";
}