1. You open the file using the open function. This returns a "file handle" through which you access the data; a file handle comprise a buffer within your program to allow the data to be read from or written to the disc in blocks, and to control whereabouts in the file you are. You should always check the return value from die to ensure that the open worked

2. If you're reading from the file, you may use the read function if you want to read up to a specified number of bytes, the <> operator in a scalar context if you want to read up to a delimiter such as the next new line, or the <> operator in a list context if you want to read the rest of the file, saving the data line by line (or in other chunks) in a list.

3. If you're writing to the file, you should specify the file handle directly after the print or printf function name, then give a list of expressions to be printed (in the case of print), or the one item to be printed followed by any parameters necessary to fill it in (in the case of printf).

4. When you finish with a file, you should close it in order to ensure that any data left in memory (via the file handle) is flushed. If you fail to close a file, Perl does it for you anyway when you end your program or reuse the file handle.

Examples

a. Reading a file, line by line

b. Reading a file, all into a single list

c. Reading a file, all into a single scalar

d. Overwriting a file, all from a single list

e. Adding to a file from a scalar


There's an alternative 3 parameter form of the open function in which you specify the open mode as an additional parameter. This form is not available in older versions of Perl.

Data read in will include new line characters that you may want to remove with chop or chomp from the scalar or lists that you've placed the data into. You may also need to add back on the new line characters at the end of each line before you write back out to file.

If you want to know why your file open failed, the error message will be in the special variable $!


See also File handling and file formatting in Perl

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