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Ctrl-L is a hidden control character
that stats for Form Feed or New Paper sheet. You can find it in the ASCII table at position 12, or better:
Dec Hex Oct Char Description
12 C 014 FF NP Form Feed, new page
You can reproduce it in a terminal using this sequence of keystrokes
where the first Ctrl-V summons the shell to display a control character. To remove it from a text file, you can either use this sequence of keystrokes or his octal/hex/decimal code taken from the ASCII table.
For example using sed you can do something like this (by typing Ctrl-V and Ctrl-L):
or by means of the octal code:
sed 's/\o14//' file
Take in mind that to actually change the content of a file using sed, you have to use the -i option. Without -i the output is sent to the terminal (standard output) which is useful for testing the command until it gives the desired result. Hope this helps.