Programming Perl, 4th Edition

readline

readline FILEHANDLE
readline

This is the internal function implementing the <FILEHANDLE> operator, but you can use it directly. The function reads the next record from FILEHANDLE, which may be a filehandle name or an indirect filehandle expression that returns either the name of the actual filehandle or a reference to anything resembling a filehandle object, such as a typeglob. In scalar context, each call reads and returns the next record until end-of-file is reached, whereupon the next call returns undef. In list context, readline reads records until end-of-file is reached and then returns a list of records. By “record”, we normally mean a line of text, but changing the value of $/ ($INPUT_RECORD_SEPARATOR) from its default value causes this operator to “chunk” the text differently. Undefining $/ makes the chunk size entire files (slurp mode).

When slurping files in scalar context, if you happen to slurp an empty file, readline returns "" the first time, and undef each subsequent time. When slurping from the magical ARGV filehandle, each file returns one chunk (again, null ...