How to use COPY command in PostgreSQL procedure with an external variable?

Creating a procedure with COPY in PostgreSQL, the FROM keyword does not recognize neither external nor internal variable.

CREATE OR REPLACE PROCEDURE YYY (
	external_variable varchar
)
language plpgsql
AS $$
declare 
internal_variable varchar;
begin
COPY TEMP_ALL_ROWS_BKMVDATA FROM external_variable DELIMITER ',' CSV HEADER;

end;
$$

response:

ERROR:  syntax error at or near "external_variable"
LINE 9: COPY TEMP_ALL_ROWS_BKMVDATA FROM external_variable DELIMITER...
                                         ^
SQL state: 42601
Character: 162

Same for internal_variable.

I've tried neglecting the variable part to make sure everything around it works and indeed.

CREATE OR REPLACE PROCEDURE YYY (
	external_variable varchar
)
language plpgsql
AS $$
declare 
internal_variable varchar;
begin
COPY TEMP_ALL_ROWS_BKMVDATA FROM '' DELIMITER ',' CSV HEADER;

end;
$$

CREATE PROCEDURE

Query returned successfully in 322 msec.

Copy command wont work in create or replace , Use execute instead

CREATE OR REPLACE PROCEDURE YYY (
    external_variable varchar
)
LANGUAGE plpgsql
AS $$
BEGIN
    EXECUTE 'COPY TEMP_ALL_ROWS_BKMVDATA FROM ''' || external_variable || ''' DELIMITER '','' CSV HEADER;';
END;
$$;