@main is an array that contains a line of the input file for each index. The program is opening the file and reading the contents into the main array in one statement. It then closes the file. It re-opens the same file for output. It loops through the array main trying to match on the string "The marker line". When it finds a match, it prints out "hello" after the match string.

Please read the perl faq on arrays and file handling. If you plan on developing, or maintaining, perl code going forward then please understand @, $, % and the difference between scalar and list context. You can go a long way with that understanding (especially if you can throw references in the mix as well).