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eof FILEHANDLE
eof ()
eof

Returns 1 if the next read on FILEHANDLE will return end of file, or if FILEHANDLE is not open. FILEHANDLE may be an expression whose value gives the real filehandle. (Note that this function actually reads a character and then ungetcs it, so isn't very useful in an interactive context.) Do not read from a terminal file (or call eof(FILEHANDLE) on it) after end-of-file is reached. File types such as terminals may lose the end-of-file condition if you do.

An eof without an argument uses the last file read. Using eof() with empty parentheses is very different. It refers to the pseudo file formed from the files listed on the command line and accessed via the <> operator. Since <> isn't explicitly opened, as a normal filehandle is, an eof() before <> has been used will cause @ARGV to be examined to determine if input is available. Similarly, an eof() after <> has returned end-of-file will assume you are processing another @ARGV list, and if you haven't set @ARGV, will read input from STDIN; see "I/O Operators" in perlop.

In a while (<>) loop, eof or eof(ARGV) can be used to detect the end of each file, eof() will only detect the end of the last file. Examples:

# reset line numbering on each input file
while (<>) {
    next if /^\s*#/;        # skip comments
    print "$.\t$_";
} continue {
    close ARGV  if eof;     # Not eof()!
}

# insert dashes just before last line of last file
while (<>) {
    if (eof()) {            # check for end of last file
        print "--------------\n";
    }
    print;
    last if eof();          # needed if we're reading from a terminal
}

Practical hint: you almost never need to use eof in Perl, because the input operators typically return undef when they run out of data, or if there was an error.