$field_start  = 8;  # 0-based
$field_length = 8;

# Get current value:
$old_value = substr($s, $field_start, $field_length);
$old_value =~ s/\s+$//;

# Change the value:
$new_value = 'bla';
substr($s, $field_start, $field_length) =
   substr($new_value . (' 'x$field_length), 0, $field_length);