You have 2 problems: wrong end condition in your for loop and you are trying to substitute from the unassigned $_ variable. No need for s/// ... just assign.

@array = ('200', '201', '205', '194', '140', '250', '280');
print "original\n" , "@array " , "\n";
$count = @array;
for ($i = 0; $i < $count; $i++) {
  if ($array[$i] gt '200') {
    $array[$i] = 200;
  }
}
print "new:\n" , "@array ", "\n";
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Tip #2 from the Basic debugging checklist: print