You have just demonstrated why haukex advised you to "post a question effectively". Your problem now has more to do with reading a file than it does with sorting. You must create a filehandle by opening (open) the file and then read from the filehandle with the diamond operatior <> (perlop). All the answers you have received use the special filehandle DATA which is open by default.

For additional info on sorting, refer to FAQ How do I sort an array by (anything)?

You'll need to open the file first. eg:

open my $in, '<', 'file.txt' or die "Cannot open file for reading: $!"
+;
print sort { ($a =~ /\S+\n$/g)[0] cmp ($b =~ /\S+\n$/g)[0] } <$in>;
close $in;
[download]

See open and close for all the details.

This worked - thanks so much :)