sub flow_count{
	
	my $HASH_OF_FLOW;
	my $view = $_[0]; # FTFM or CTFM

        ...
        ...
        foreach my $file(@FILES_IN_THE_FOLDER){
           my $key = "$reg;$date";
           # M2 is a number which has been computed
	   $HASH_OF_FLOW->{'$key'} = 'M2';
        }

     return $HASH_OF_FLOW; 
}

my $FTFM_HASH_OF_FLOW = flow_count("FTFM");
my $CTFM_HASH_OF_FLOW = flow_count("CTFM");
my @FTFM_KEYS = keys %$FTFM_HASH_OF_FLOW;


foreach my $key(@FTFM_KEYS){
	my $FTFM_CTFM_diff = $FTFM_HASH_OF_FLOW{$key} - $CTFM_HASH_OF_FLOW{$key};
	print $OUTFILE "$key;$FTFM_HASH_OF_FLOW{$key};$CTFM_HASH_OF_FLOW{$key};$FTFM_CTFM_diff\n";
}