Hello Monks, I could merge the input files in row wise, i am not able to merge it in column wise, as expected, Please help.

Below code i wrote which will merge row wise

use strict;
use warnings;
opendir(DIR, '.') or die "Can't open .$!";
my @inputfiles = sort {$a <=> $b} (grep(/\.txt$/,readdir(DIR)));
my $Outputfilename="Output.txt";
my $file;
print @inputfiles;
open (OUTPUT,">$Outputfilename");
foreach $file (@inputfiles)
{
open (INPUT,$file);
while(<INPUT>)
{
print OUTPUT $_;
}
}
close(OUTPUT);