An eof without an argument uses the last file read as argument. Empty parentheses () may be used to indicate the pseudo file formed of the files listed on the command line, i.e., eof() is reasonable to use inside a while (<>) loop to detect the end of only the last file. Use eof(ARGV) or eof without the parentheses to test EACH file in a while (<>) loop. Examples:

    # reset line numbering on each input file
    while (<>) {
        print "$.\t$_";
        close(ARGV) if (eof);   # Not eof().
    }

    # insert dashes just before last line of last file
    while (<>) {
        if (eof()) {
            print "--------------\n";
            close(ARGV);        # close or break; is needed if we
                                # are reading from the terminal
        }
        print;
    }

Practical hint: you almost never need to use eof in Perl, because the input operators return undef when they run out of data.