08:48:45;"";"";;"";
08:49:43;"DAL11";"08:47";527;"08:50";530
08:50:41;"";"";;"";
08:51:40;"";"";;"";
08:52:38;"";"";;"";
08:53:36;"ACA879";"08:51";531;"08:53";533
08:54:35;"";"";;"";
08:55:33;"SHT8K";"08:48";528;"08:55";535
08:56:31;"";"";;"";
08:57:30;"DLH9FM";"08:53";533;"08:57";537


##</code><code>##

#!/usr/bin/perl
use strict;
use warnings;
use diagnostics;


#################  IN WHICH DIRECTORY WE ARE ########################

my $Current_Dir = `pwd`;
print STDOUT "the current directory is $Current_Dir";

#####################################################################

################### OPEN THE FIRST INFILE ############################
# open the first file 
# do not forget the "" to declare  my ${first_file} = 

my ${first_file} = "$ARGV[0]";
open(INFILE,"<${first_file}") or die "Can't open ${first_file} : $!";

#################################################################


################### OPEN THE OUTFILE ############################
# name of the OUTFILE
# never put a \n at the end of the OUTFILE name otherwise it does not create the output
 my ${outfile_name} = "Analysis_${first_file}";

# to open the file
# OUTFILE is the name of the HANDLE in this case
  open (OUTFILE, ">${outfile_name}") or die "Can't open ${outfile_name}: $!";

##################################################################

	while (<INFILE>){


	chomp($_);
	my @Parts = split(";");
	my $aircraft_id = $Parts[1];
	my $ATO_min = $Parts[3];
	my $CTO_min = $Parts[5];
	my $ATO = $Parts[2];
	my $CTO = $Parts[4];

	my $delay_ATO_CTO_min;

	if (($CTO =~ /(\d\d):(\d\d)/) & ($ATO =~ /(\d\d):(\d\d)/)){
		$delay_ATO_CTO_min = $CTO_min-$ATO_min;
		}
		else{
		$delay_ATO_CTO_min = "";
		}



	my $flight_big_delay;

		# condition on the delay
		if ($delay_ATO_CTO_min > 10){
		$flight_big_delay = $aircraft_id;
		print OUTFILE "$aircraft_id;$ATO_min;$CTO_min;$delay_ATO_CTO_min;$flight_big_delay\n";
		}
		else{
		print OUTFILE "$aircraft_id;$ATO_min;$CTO_min;$delay_ATO_CTO_min\n";
		}
	
	}
close INFILE;
close OUTFILE;


##</code><code>##

"";;;
"DAL11";527;530;3
"";;;
"";;;
"";;;
"ACA879";531;533;2
"";;;
"SHT8K";528;535;7
"";;;
"DLH9FM";533;537;4


##</code><code>##

Argument "" isn't numeric in numeric gt(>) at Monks_Treatment.pl (that's the name of the program) line 60, <INFILE>