What is your data?

Win8 Strawberry 5.8.9.5 (32)  Tue 10/13/2020 15:55:21
C:\@Work\Perl\monks\pragovnj
>cat junque.txt
foo bar CAST(CAST(hs_date AS DATE FORMAT 'YYYYMMDD') AS CHAR(08)) as h
+s_date biz boz

C:\@Work\Perl\monks\pragovnj
>perl -p -i.bak -e "s/\QCAST(CAST(hs_date AS DATE FORMAT 'YYYYMMDD') A
+S CHAR(08)) as hs_date\E/FORMAT_DATE(hs_date, 'YYYYMMDD') as hs_date/
+g;" junque.txt

C:\@Work\Perl\monks\pragovnj
>cat junque.txt
foo bar FORMAT_DATE(hs_date, 'YYYYMMDD') as hs_date biz boz

C:\@Work\Perl\monks\pragovnj
>cat junque.txt.bak
foo bar CAST(CAST(hs_date AS DATE FORMAT 'YYYYMMDD') AS CHAR(08)) as h
+s_date biz boz
[download]

(Sorry for any wrap-around. :) In general, "it doesn't work" is a very poor problem description.

Update: Please see Short, Self-Contained, Correct Example for a discussion of good problem presentation practices.