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use strict;
use warnings;

open $USER1, '<', $userfile1 or die "Couldn't open file: $userfile1 - 
+$!";
...
while(<$USER1>) { ...

open $OUT, '>', "CONCATENATED_FILES";
...
open $USER1, '<', $userfile1;

use autodie;

use strict;
use warnings;
...
    last if !defined $file1[$i] or !defined $file2[$i];
    print "$file1[$i]\t$file2[$i]\n";
}