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8.11. Processing Binary Files
8.11.1. Problem
You want to read 8-bit binary data as 8-bit binary data, i.e., neither as characters in a particular encoding nor as a text file with any newline or end-of-file conversions that your I/O library might want to do.
8.11.3. Discussion
The binmode function lets you specify new I/O layers for a filehandle. The default layer to specify is :raw, which removes any layers that would interfere with binary data. The Solution is thus equivalent to:
binmode(HANDLE, ":raw");
except that explicitly specifying :raw only works on Perl 5.8 and later. The one-argument form of binmode works on all versions of Perl.
Because Perl makes :crlf the default if you are on an operating system that needs it, you should rarely (if ever) need to specify :crlf in your program. Furthermore, it's generally not wise to add or remove the :crlf layer once you've begun reading from the file, as there may be data already read into buffers that you can't unread. You can, however, safely change the :encoding(...) layer midstream (when parsing XML, for example).
You should get into the habit of calling binmode when you open a binary file. This will make your program portable to systems that might (un)helpfully translate bytes in your binary file into something unusable.
You may specify the I/O layers when you open a filehandle, rather than using binmode after the fact:
open(FH, "< :raw", $filename); # binary mode
Specify the default set of layers for all subsequently opened input and output filehandles with the open pragma:
use open IN => ":raw"; # binary files
8.11.4. See Also
The PerlIO(3) manpage; the open and binmode functions in perlfunc(1) and in Chapter 29 of Programming Perl; your system's open(2) and fopen(3) manpages
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| 8.10. Removing the Last Line of a File |  | 8.12. Using Random-Access I/O |
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