Dal manuale perl (perlop)
codice:
 <<EOF   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 regu-
               lar quoting.  An unquoted identifier works like double quotes.
               There must be no space between the "<<" and the identifier,
               unless the identifier is quoted.  (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.

                      print <<EOF;
                   The price is $Price.
                   EOF

                      print << "EOF"; # same as above
                   The price is $Price.
                   EOF

                      print << ‘EOC‘; # execute commands
                   echo hi there
                   echo lo there
                   EOC

                      print <<"foo", <<"bar"; # you can stack them
                   I said foo.
                   foo
                   I said bar.
                   bar

                      myfunc(<< "THIS", 23, <<’THAT’);
                   Here’s a line
                   or two.
                   THIS
                   and here’s another.
                   THAT

               Just don’t forget that you have to put a semicolon on the end
               to finish the statement, as Perl doesn’t know you’re not going
               to try to do this:

                      print <<ABC
                   179231
                   ABC
                      + 20;

               If you want your here-docs to be indented with the rest of the
               code, you’ll need to remove leading whitespace from each line
               manually:

                   ($quote = <<’FINIS’) =~ s/^\s+//gm;
                      The Road goes ever on and on,
                      down from the door where it began.
                   FINIS

               If you use a here-doc within a delimited construct, such as in
               "s///eg", the quoted material must come on the lines following
               the final delimiter.  So instead of

                   s/this/<<E . ’that’
                   the other
                   E
                    . ’more ’/eg;

               you have to write

                   s/this/<<E . ’that’
                    . ’more ’/eg;
                   the other
                   E

               If the terminating identifier is on the last line of the pro-
               gram, you must be sure there is a newline after it; otherwise,
               Perl will give the warning Can’t find string terminator "END"
               anywhere before EOF....

               Additionally, the quoting rules for the identifier are not
               related to Perl’s quoting rules -- "q()", "qq()", and the like
               are not supported in place of '' and "", and the only interpo-
               lation is for backslashing the quoting character:

                   print << "abc\"def";
                   testing...
                   abc"def

               Finally, quoted strings cannot span multiple lines.  The gen-
               eral rule is that the identifier must be a string literal.
               Stick with that, and you should be safe.
Quindi quella espressione significa:
Carica nella stringa $stringa tutto quanto e` contenuto tra la riga successiva a "HEREDOC" alla riga precedente quella che inizia con HEREDOC.

Per vedere qualcosa dovresti aggiungere al tuo pezzo di programma:
print "$stringa\n";