This will do it:

use Data::Dumper;
                                                                      
+          
open(IN, "< myfile.txt") or die $!

my $data_ok = 0;
                                                                      
+          
while(<IN>) {
  chomp;
                                                                      
+          
  m/^\s*>\s*$/ and last;       # stop when encountering that last ">".

  $data_ok and push @data, $_; # keep all data inbetween.

  m/Testnok/ and $data_ok = 1; # start after "Testnok".
}
print Dumper \@data;
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Cheers, Sören