Remove string from file

perl -ni -e 'print unless /pattern/' /path/to/filename

Replace string with string

perl -p -i -e 's/PATTERN/NEW_ENTRY/g' /path/to/filename

Insert string where delimeter is found
This will replace the original delimiter with the new entry and then add the delimiter so you can use it again in the future.

perl -p -i -e 's/DELIMETER/NEW_ENTRY\n\nDELIMETER/g' /path/to/filename

This will grab anything thats in the standard IPv4 format (anything! not always IPs…but usually).

perl -ne 'print "$&\n" while m#\d+\.\d+\.\d+\.\d+#g' file.txt

Search for and find whole paragraphs containing $string

perl -00ne "print if /$string/i" < file.txt