In case the many replies using this approach have not made it clear, you can get more information about $/ from running "perldoc perlvar", and scanning down till you see this one described (with the long name "$INPUT_RECORD_SEPARATOR"). Normally -- i.e. by default -- $/ = "\n" (which in reality would be "\r\n" on MS-DOS/Windows systems, "\r" on "Classic" macs, and truly "\n" on unixes).

It's the character pattern that is removed from the end of a string when you "chomp" the string, and it is the pattern that the diamond operator looks for when reading data from a file handle into a scalar, to know when to stop. If set to "undef", it causes a single read operation to absorb the entire file and assign it all to a single scalar.