====== Perl Quick Reference ======
  * ($): variable is a name for a container that holds one or more values; use $ to get the value.
  * (!): negation operator
  * (tr): transliterating one group of characters into another.
  * (s): substitution 
  * (%): modulus operator: 
  * \s : shortcut for whitespace
  * \s+ : match for one or more whitespace characters
  * /g : modifier-global
  * /d : modifier-delete found but unreplaced characters
  * == : equal
  * = : assignment operator  
  * . : string values can be concatenated with the . operator
  * ( * ): match the preceding item 0 or more times 
  * ( ^ ): the caret match the beginning of a line
  * $ : match the end of a line
  
===== installation =====
  * summary of configuration
    * ''perl -V''
    * including the content of ''@INC''
  * locate specific modules
    * ''perldoc -l Module::Name''; for example, ''perldoc -l Bio::SearchIO''