What do I have to do to this file so that it is ‘compatible’ to UNIX?

As others have explained, the basic issue is to remove the carriage-return characters (a.k.a. "\r", "\x0d", "\015" and the keycode emitted by 'ctrl-M') which Windows puts in front of every line-feed character.

Of course, a Windows plain-text file with the carriage-returns still present is also "compatible" with UNIX, for a fairly wide range of activities where the presence of carriage-returns makes no difference to the user or the application.

In the context of applications written in Perl, whenever I write Perl for a task where the range of inputs might include Windows text files as well as linux/unix text data, and where the nature of the line termination might be an issue, I just use the following instead of chomp:

s/[\r\n]+$//;
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That way, the script will behave consistently on both Windows and linux/unix systems, regardless whether the data is coming from the same system or a different system.