Very nice, concise overview. A couple of additional notes to keep in mind regarding here docs:

• The end tag specifier must follow the << without any intermediate space.
  
  If the end tag is quoted explicitly (with either single or double quotes), spaces may be present:
  

      print <<  "ENDTAG"; # works
      print <<  'ENDTAG'; # works
      print <<   ENDTAG;  # syntax error!
  [download]

• Any leading spaces in the text of the here doc will be printed as such, which means using here docs in indented blocks gets messy:
  
  Non-indented code block
  

  print "printing here doc:\n";
  print <<"ENDTAG";
  This text has 0 leading spaces
      This text has 4 leading spaces
  ENDTAG
  
  OUTPUT:
  printing here doc:
  This text has 0 leading spaces
      This text has 4 leading spaces
  [download]

  Indented code block (same output as above)
  

  {
      print "printing here doc:\n";
      print <<"ENDTAG";
  This text has 0 leading spaces
      This text has 4 leading spaces
  ENDTAG
  }
  [download]

  But, if the printed text includes leading whitespace, spaces can be included as part of the end tag to clean up the code block:

  {
      print "printing here doc:\n";
      print <<"    ENDTAG";
      This text has 4 leading spaces
      ENDTAG
  }
  
  OUTPUT:
  printing here doc:
      This text has 4 leading spaces
  [download]

• The Filter::Indent::HereDoc and Filter::HereDocIndent modules allow here docs to be used within indented code blocks without adding the leading whitespace from the indentation to the output.

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