Assumes that file1.txt and file2.txt contain the same number of lines.

  #! /usr/bin/perl -w
  use strict;

  open (my $file1, '<', 'file1.txt') or die $!;
  open (my $file2, '<', 'file2.txt') or die $!;

  while (my $line1 = <$file1>)
  {
    chomp $line1;
    my $line2 = <$file2>;
    print $line, ' ', $line2;
  }

  close $file1;
  close $file2;
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Update: Code correction.