I might be misunderstanding the question, but if not, I think the solution is as easy as this:

use strict;

my $countline = 0;
my $mytest;

my @array = (); #<- added line

open (MYFILE,"<testing.txt")||die "Could not open input file\n";

while($mytest = <MYFILE>) {

                   
         my @datas= split (/\s+/, $mytest);
         


       if ($datas[0] eq "Hello")
            {

          $countline++;
          $array[$countline] = $countline; #<- added line

            }

my @test1 = $countline;
print $test1[0];
}
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