#! perl -slw
use strict;
use Data::Dump qw[ pp ];

my @lines;

for my $file ( qw[ x1.js x2.js y1.js y2.js z1.js z2.js ] ){
    open FH, '<', $file or die $!;
    push @{ $lines[ $. ] }, $_ while <FH>;
    close FH;
}

for my $l ( @lines ) {
    next unless defined $l;
    print @$l;
}