|P is for Practical|
chomp does exactly what the other posters say - removes the current record separator at the end of the line where the record separator is stored in the special variable $/
That variable defaults to the EOL (end of line) character for your platform.
So if you run your script on a Unix type OS, only the newline will be removed (since it's expected that on that OS files will end in only a newline).
If you run your script on Windows, the carrage return and newline will both be removed (since it's expected that under Windows files will end in both a carriage return and a newline).
This means that when you chomp a line on a file made on the same OS as where your script is running, the correct thing will happen without your intervention.
If, on the other hand, you want to run a script that will strip the EOL marker off a file regardless of what OS the script is running on and whether that file came from Unix or Windows, you need to do that yourself, with something like:
This works easily due to the neat fact that \r\n and \n both end in \n so the <> operator is going to split Windows and Unix files into lines correctly and all you have to do is strip off the trailing \n and optionally a preceding \r.
Of course the other EOL possibility is that of a Classic MacOS text file, where the EOL marker is a lone carriage return (ie. \r). Mercifully you won't come across these files nearly as much as you used to and dealing with them is an exercise left to the reader ;)