It is written in the POD of perldata (version 5, release 6):
A line-oriented form of quoting is based on the shell "here-document" syntax. Following a << you specify a string to terminate the quoted material, and all lines following the current line down to the terminating string are the value of the item. The terminating string may be either an identifier (a word), or some quoted text. If quoted, the type of quotes you use determines the treatment of the text, just as in regular quoting. An unquoted identifier works like double quotes. There must be no space between the << and the identifier. (If you put a space it will be treated as a null identifier, which is valid, and matches the first empty line.) The terminating string must appear by itself (unquoted and with no surrounding whitespace) on the terminating line.
Granted it's a little oblique, but it is there--as someone once reminded me. Since then, I have seen it mentioned in a couple of other places, including (if memory serves) Rat, Camel (currently 50% off, via that link), and the Spikey Ball.