How to replace part of a text file between markers with another text file?

Say I have a text file like this:

# custom content section
a
b

### BEGIN GENERATED CONTENT
c
d
### END GENERATED CONTENT

I'd like to replace the portion between the GENERATED CONTENT tags with the contents of another file.

What's the simplest way to do this?

Solution 1:

lead='^### BEGIN GENERATED CONTENT$'
tail='^### END GENERATED CONTENT$'
sed -e "/$lead/,/$tail/{ /$lead/{p; r insert_file
        }; /$tail/p; d }"  existing_file

Solution 2:

newContent=`cat new_file`
perl -0777 -i -pe "s/(### BEGIN GENERATED CONTENT\\n).*(\\n### END GENERATED CONTENT)/\$1$newContent\$2/s" existing_file