use strict;
use warnings;
use Tie::File;

opendir IN, 'Output'; # name of folder where files are
my @in = grep { /\.txt$/ } readdir IN; # read all files in dir
closedir IN;

for my $in (@in) {
open IN, '<', "Output/$in" || next;

tie my @array, 'Tie::File', $in or die "Could not open file '$in' $!";
$array[0] = 'Header1, Header2, Header3, etc etc.';
untie @array;
}