Use push(@array, $mytest); where you currently count:

use strict;
use warnings;

my $mytest;
my @array;

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

while ($mytest = <MYFILE>) {
   my @datas = split(/\s+/, $mytest);

   push(@array, $mytest)         
      if $datas[0] eq "Hello";
}

# my $countline = @array;

print("The matching lines:\n");
foreach my $line (@array) {
   print($line);
}
[download]

use strict;
use warnings;

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

my @array = grep {
               my @datas = split(/\s+/, $_);
               $datas[0] eq "Hello"
            }
            <MYFILE>;

# my $countline = @array;

print("The matching lines:\n");
foreach my $line (@array) {
   print($line);
}
[download]