use feature ':5.10';

open (KEYFILE, "arr2.txt");
    @arr2 = <KEYFILE>;
close(KEYFILE);

# Declare and fill up the hash
my %elements;
foreach (@arr2) {
    $elements{$_} = 1;
};

#I'm repeating this for several quarterly files:
for($year=2006; $year<2012; $year=$year+1){
    for($i=1; $i<5; $i=$i+1){
    
        # Load each quarterly file
        $filea = $year . "QTR" . $i . "arr1.txt";
        open(MYINFILE, $filea);
            @arr1 = <MYINFILE>;
        close(MYINFILE);
        $sizegn = @arr1;

        # For each entry in the quarterly file
        for($j=0; $j<$sizegn; $j++){
            
            # Pick only lines that contain the string "txt"
            if($arr1[$j] =~ m/txt/){
            
                # splits the elements of @arr1, and format the first e
+lement into 7-digit number:
                @arraydata = split(/\|/, $arr1[$j]); 
                my $element = sprintf("%07d", $arraydata[0]); 
                
                # now check if $element is in hash :
                if (exists $elements{$element}){
                    open(MYOUTFILE, ">>Arr3.dat");
                    print MYOUTFILE $arr1[$j];
                    close(MYOUTFILE);
                }
                print "D'oh - '$element' not found\n" unless (exists $
+elements{$element});
            }
        }
    }
}