G'day saro,

Welcome to the Monastery.

I suspect there's more to this than you're telling us; however, based on what you have provided, I'd probably aim for a technique such as this:

#!/usr/bin/env perl

use strict;
use warnings;

my $flag = 'NINE';

while (<DATA>) {
    print if (split)[0] eq $flag;
}

__DATA__
DATA.TXT
==============
ONE TWO THREE FOUR FIVE SIX SEVEN EIGHT
NINE TEN ELEVEN TWELEVE 12-MAR-2020 /net/slcnas22/scratch/domain_name 
+http://testurl/home username_pwd
THIRTEEN FOURTEEN FIFTEEN
SIXTEEN SEVENTEEN EIGHTEEN NINETEEN TWENTY
[download]

Output:

NINE TEN ELEVEN TWELEVE 12-MAR-2020 /net/slcnas22/scratch/domain_name 
+http://testurl/home username_pwd
[download]

I see that ++Discipulus has provided a number of good documentation links; although, this technique does not require chomp nor does it use regexes. I also suggest you look at the autodie pragma: I highly recommend it.