use strict; 
use warnings; 
open my $A_IN,  '<', 'a.txt' or die "Cannot open a.txt\n"; 
open my $B_IN,  '<', 'b.txt' or die "Cannot open b.txt\n"; 
open my $C_OUT, '>', 'c.txt' or die "Cannot open c.txt\n"; 
while (1) { 
    my $a_line =  <$A_IN>; 
    my $b_line =  <$B_IN>; 
    last if !$a_line and !$b_line; 
    die "Input files are not the same length\n" if $a_line xor $b_line; 
    chomp $a_line; 
    chomp $b_line; 
    my $field_a0 = (split /,/, $a_line)[0]; 
    my $field_b0 = (split /,/, $b_line)[0]; 
    print {$C_OUT} "$field_a0,$field_b0\n"; 
} 
close $C_OUT; 
close $B_IN; 
close $A_IN;

my @b_data; 
open my $B_IN,  '<', 'B.txt' or die "Cannot open B.txt\n"; 
while (<$B_IN>){ 
    push @b_data, [split /,/, $_]; 
} 
close $B_IN;

    foreach (@b_data) { 
        if ($_->[0] == $a_line) { 
            $_->[1] = $TODAY; 
        } 
    }