Thank you for the reply Rolf! I'm guessing I should've put all the code instead of a little snippet, as it isn't very long.

use strict;
use warnings;
use File::Slurp;

my $countfile = 1;

open (IN, 'C:\terms.txt');
my @terms = <IN>;
close IN;

open (OUT,'>>C:\result.txt');

while ($countfile < 10000) {
    
    my $text = read_file('C:\file' . $countfile . '.xml') or die "Can'
+t open file!";
            
        if($text =~ /upperboundary(.+?)lowerboundary/s){

        if(grep {$_ eq $1} @terms) {
            
        print OUT "$1\t";
        
        }
    }
    
    close IN;
    $countfile++;
    open (IN, 'C:\file' . $countfile . '.xml') or die "End of files!";
}

close OUT;
[download]

I can see the array populated if I print it, but there must be some other thing that's destroying the output at the middle...