IF utl_file.is_open(input_file) THEN
     LOOP
     BEGIN         
         utl_file.get_line(input_file, w_newline);
         w_lcnt := w_lcnt + 1;
         IF SUBSTR(w_newline,46,2) in ('02','03') THEN
           w_ProcessLine := substr(w_newline,1,142);
           dbms_output.put_line(w_ProcessLine);

select distinct client_charset from v$session_connect_info where sid=sys_context('userenv','sid');

SELECT parameter, value FROM v$nls_parameters WHERE parameter IN ('NLS_LANGUAGE','NLS_TERRITORY','NLS_CHARACTERSET');

select distinct client_charset from v$session_connect_info where sid=sys_context('userenv','sid');

$ NLS_LANG=AMERICAN_AMERICA.WE8MSWIN1252; export NLS_LANG

set serveroutput on
Declare
  w_input_dir             VARCHAR2(100);
  w_input_file            VARCHAR2(50);
  w_newline               VARCHAR2(2000);
  w_ProcessLine           VARCHAR2(200);
  w_lcnt                  NUMBER := 0;
  w_strlgth               NUMBER;
  w_chrasci               NUMBER;
  erm                     VARCHAR2(50):= '';
  input_file      utl_file.file_type;
  
  
BEGIN  
 dbms_output.enable(null);
    
    w_input_dir      := 'WCPOLS_INPUT_DIR';--'CENGROUP_OUTPUT';
    w_input_file     := 'Invalid_ASCII_characters_out.txt';    

    input_file    := utl_file.fopen(w_input_dir, w_input_file, 'r',32000);

    
    IF utl_file.is_open(input_file) THEN
     LOOP
     BEGIN         
         utl_file.get_line(input_file, w_newline);
         w_lcnt := w_lcnt + 1;
         IF SUBSTR(w_newline,46,2) = '02' THEN
           w_ProcessLine := convert(substr(w_newline,1,142),'WE8MSWIN1252','UTF8');
           select length(w_ProcessLine) into w_strlgth from dual;           
            for i in 1..w_strlgth loop
              if ascii(substr(w_ProcessLine,i,1)) between 128 and 255 then 
                dbms_output.put_line('Invalid Character '||chr(ascii(substr(w_ProcessLine,i,1)))||' at line: '||w_lcnt ||' and '||
                                     'column pos:'||i);
              end if;
            end loop;
          ELSIF SUBSTR(w_newline,46,2) = '03' THEN
                w_ProcessLine := convert(substr(w_newline,1,110),'WE8MSWIN1252','UTF8');
                select length(w_ProcessLine) into w_strlgth from dual;
              for i in 1..w_strlgth loop
                  if ascii(substr(w_ProcessLine,i,1)) between 128 and 255 then 
                  dbms_output.put_line('Invalid Character '||chr(ascii(substr(w_ProcessLine,i,1)))||' at line: '||w_lcnt ||' and '||
                                       'column pos:'||i);
                end if;
              end loop;
         END IF;
     
     EXCEPTION WHEN no_data_found THEN
         EXIT;
     END;
     END LOOP; --main loop
     
    END IF;  --InputFile is_open

   utl_file.fclose(input_file);
   

END ; 
/

The output is correct as-
Invalid Character à at line: 6 and column pos:67
Invalid Character © at line: 6 and column pos:68
Invalid Character ½ at line: 1485 and column pos:56

Now when I run the above code on Linux box by setting NLS_LANG variable to UT8 as
[hpatil@AUS-VX-APWX-TS1 sql]$ setenv NLS_LANG AMERICAN_AMERICA.UTF8
[hpatil@AUS-VX-APWX-TS1 sql]$ sqlplus
the output is
SQL> @Code1.sql
Invalid Character à at line: 6 and column pos:67
Invalid Character © at line: 6 and column pos:68
Invalid Character ½ at line: 1485 and column pos:56

now why the  is coming automatically with symbols © and ½. Any Idea what I am still missing? Please advise.