use strict;
use warnings;

$/ = \20_000_000; #I've just split a 20-meg file
open my $f1, "<", '1' or die $!; # piece 1 is named 1
binmode $f1;
open my $f2, "<", '2' or die $!; # piece 2 is named 2
binmode $f2;
while(my $x = <$f1>){
	while(my $y = <$f2>){
	open my $out, ">", "outfile" or die $!;
    binmode $out;
    print $out $x.$y;
	}
}


##</code><code>##

use strict;
use warnings;

$/ = \20_000_000; #I've just split a 20-meg file into 2 pieces
open my $f1, "<", '1' or die $!; piece 1 is named 1
binmode $f1;
open my $f2, "<", '2' or die $!; # piece 2 is named 2
binmode $f2;
while(defined(my $x = <$f1>) && (my $y = <$f2>)){
	open my $out, ">", "outfile" or die $!;
    binmode $out;
    print $out $x.$y;
}