This one-liner even creates a back-up in case things go wrong!

>perl -pibak -e"$_ = qq(foo_1\nfoo_2\nfoo_3\nfoo_4\n) if /^foo_$/" xx.
+txt

>dir xx.txt*
 Volume in drive C is Windows
 Volume Serial Number is AE03-D45F

 Directory of C:\Users\Bill\forums\monks

03/09/2021  10:47 PM                57 xx.txt
03/09/2021  07:04 PM                35 xx.txtbak
               2 File(s)             92 bytes
               0 Dir(s)  379,932,401,664 bytes free

>type xx.txt
hello
world
foo_1
foo_2
foo_3
foo_4
foo
abc
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