use strict;
use warnings;
use Getopt::Long;
use Algorithm::Cron;

my $file;
my $howmany;
my $crontab;
my $help;
my @lines;

my $helptext = "USAGE:\n $0 [ [-f filename | -c STRING] -n N]\n\n".
			" $0 -f filename\n".
			" $0 -f filename -n 2\n".
			" crontab -l | $0 \n".
			" crontab -l | $0 -n 2\n".
			" cat filename | $0\n".
			" cat filename | $0 -n 3\n".
			" $0 -c 'crontab entry'\n".
			" $0 -c 'crontab entry' -n 5\n".
			" $0  (paste some content followed by CTRL-D or CTRL-Z on a newline)\n".
			" $0  -n 4 (paste some content followed by CTRL-D or CTRL-Z on a newline)\n".
			" $0 -h (print this help)\n";
			
GetOptions ("f|file=s" => \$file,
            "n=i"   => \$howmany,
			"c|crontab=s"	=> \$crontab,
            "h|help"  => \$help)  
	or die( $helptext );

print $helptext and exit if $help;	

if ( $crontab )	{ @lines = $crontab }
elsif ( $file and -e -f -r $file){
	open my $fh, '<', $file or die;
	@lines = <$fh>;
}
else{ @lines = <> }

foreach my $line ( 
			sort { 
				Algorithm::Cron->new( base => 'local',
				crontab => join' ',(split /\s+|\t/,$a)[0..4])->next_time(time)
				<=>
				Algorithm::Cron->new( base => 'local',
				crontab => join' ',(split /\s+|\t/,$b)[0..4])->next_time(time)
			} 
			grep { /^(\d|\*)/ } @lines 
){
	my @parts = split /\s+|\t/,$line;
	my $now = time;
	my $repeat = $howmany;
	print scalar localtime ( Algorithm::Cron->new( base => 'local',
							crontab => join' ', @parts[0..4])->next_time($now)
	);
	print " => ( @parts[0..4] )",($crontab ? "" : " => @parts[5..$#parts]"),"\n";
	if ( --$repeat ){
		while( $repeat > 0){
			$now = Algorithm::Cron->new( base => 'local',
						crontab => join' ', @parts[0..4])->next_time( $now );
			print scalar localtime ( Algorithm::Cron->new( base => 'local',
							crontab => join' ', @parts[0..4])->next_time($now)
			);
			print "\n";
			$repeat--;		
		}
	}
}



##</code><code>##

cat crontab.txt

# Crontab Environmental settings
SHELL=/bin/bash
PATH=/sbin:/bin:/usr/sbin:/usr/bin
MAILTO=root
00 3 * 7 0 /path/to/command
#15 20 * 1-7 * /path/to/command2
*/30 7,21 1-15 1 * /path/to/another/command
# m h dom mon dow user  command
21 *    * * *   root    cd / && run-parts --report /etc/cron.hourly
0,30 6    * * *   root    test -x /usr/sbin/blah
#47 6    * * 7   root    test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.weekly )
52 6    1 * *   root    test -x /usr/sbin/blahblah || ( cd / && run-parts )


cat crontab.txt | perl cron-explain.pl

Tue Nov 27 14:21:00 2018 => ( 21 * * * * ) => root cd / && run-parts --report /etc/cron.hourly
Wed Nov 28 06:00:00 2018 => ( 0,30 6 * * * ) => root test -x /usr/sbin/blah
Sat Dec  1 06:52:00 2018 => ( 52 6 1 * * ) => root test -x /usr/sbin/blahblah || ( cd / && run-parts )
Tue Jan  1 07:00:00 2019 => ( */30 7,21 1-15 1 * ) => /path/to/another/command
Sun Jul  7 03:00:00 2019 => ( 00 3 * 7 0 ) => /path/to/command


cat crontab.txt | perl cron-explain.pl -n 3

Tue Nov 27 14:21:00 2018 => ( 21 * * * * ) => root cd / && run-parts --report /etc/cron.hourly
Tue Nov 27 15:21:00 2018
Tue Nov 27 16:21:00 2018
Wed Nov 28 06:00:00 2018 => ( 0,30 6 * * * ) => root test -x /usr/sbin/blah
Wed Nov 28 06:30:00 2018
Thu Nov 29 06:00:00 2018
Sat Dec  1 06:52:00 2018 => ( 52 6 1 * * ) => root test -x /usr/sbin/blahblah || ( cd / && run-parts )
Tue Jan  1 06:52:00 2019
Fri Feb  1 06:52:00 2019
Tue Jan  1 07:00:00 2019 => ( */30 7,21 1-15 1 * ) => /path/to/another/command
Tue Jan  1 07:30:00 2019
Tue Jan  1 21:00:00 2019
Sun Jul  7 03:00:00 2019 => ( 00 3 * 7 0 ) => /path/to/command
Sun Jul 14 03:00:00 2019
Sun Jul 21 03:00:00 2019


perl cron-explain.pl -c "2-5 9 12 4 *" -n 4

Fri Apr 12 09:02:00 2019 => ( 2-5 9 12 4 * )
Fri Apr 12 09:03:00 2019
Fri Apr 12 09:04:00 2019
Fri Apr 12 09:05:00 2019