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  CONTEXTS  SIGILS  ref        ARRAYS        HASHES
  void      $scalar SCALAR     @array        %hash
  scalar    @array  ARRAY      @array[0, 2]  @hash{'a', 'b'}
  list      %hash   HASH       $array[0]     $hash{'a'}
            &sub    CODE
            *glob   GLOB       SCALAR VALUES
                    FORMAT     number, string, ref, glob, undef
  REFERENCES
  \      reference       $$foo[1]       aka $foo->[1]
  $@%&*  dereference     $$foo{bar}     aka $foo->{bar}
  []     anon. arrayref  ${$$foo[1]}[2] aka $foo->[1]->[2]
  {}     anon. hashref   ${$$foo[1]}[2] aka $foo->[1][2]
  \()    list of refs
                         SYNTAX
  OPERATOR PRECEDENCE    foreach (LIST) { }     for (a;b;c) { }
  ->                     while   (e) { }        until (e)   { }
  ++ --                  if      (e) { } elsif (e) { } else { }
  **                     unless  (e) { } elsif (e) { } else { }
  ! ~ \ u+ u-            given   (e) { when (e) {} default {} }
  =~ !~
  * / % x                 NUMBERS vs STRINGS  FALSE vs TRUE
  + - .                   =          =        undef, "", 0, "0"
  << >>                   +          .        anything else
  named uops              == !=      eq ne
  < > <= >= lt gt le ge   < > <= >=  lt gt le ge
  == != <=> eq ne cmp ~~  <=>        cmp
  &
  | ^             REGEX MODIFIERS       REGEX METACHARS
  &&              /i case insensitive   ^      string begin
  || //           /m line based ^$      $      str end (bfr \n)
  .. ...          /s . includes \n      +      one or more
  ?:              /x /xx ign. wh.space  *      zero or more
  = += last goto  /p preserve           ?      zero or one
  , =>            /a ASCII    /aa safe  {3,7}  repeat in range
  list ops        /l locale   /d  dual  |      alternation
  not             /u Unicode            []     character class
  and             /e evaluate /ee rpts  \b     boundary
  or xor          /g global             \z     string end
                  /o compile pat once   ()     capture
  DEBUG                                 (?:p)  no capture
  -MO=Deparse     REGEX CHARCLASSES     (?#t)  comment
  -MO=Terse       .   [^\n]             (?=p)  ZW pos ahead
  -D##            \s  whitespace        (?!p)  ZW neg ahead
  -d:Trace        \w  word chars        (?<=p) ZW pos behind \K
                  \d  digits            (?<!p) ZW neg behind
  CONFIGURATION   \pP named property    (?>p)  no backtrack
  perl -V:ivsize  \h  horiz.wh.space    (?|p|p)branch reset
                  \R  linebreak         (?<n>p)named capture
                  \S \W \D \H negate    \g{n}  ref to named cap
                                        \K     keep left part
  FUNCTION RETURN LISTS
  stat      localtime    caller         SPECIAL VARIABLES
   0 dev    0 second      0 package     $_    default variable
   1 ino    1 minute      1 filename    $0    program name
   2 mode   2 hour        2 line        $/    input separator
   3 nlink  3 day         3 subroutine  $\    output separator
   4 uid    4 month-1     4 hasargs     $|    autoflush
   5 gid    5 year-1900   5 wantarray   $!    sys/libcall error
   6 rdev   6 weekday     6 evaltext    $@    eval error
   7 size   7 yearday     7 is_require  $$    process ID
   8 atime  8 is_dst      8 hints       $.    line number
   9 mtime                9 bitmask     @ARGV command line args
  10 ctime               10 hinthash    @INC  include paths
  11 blksz               3..10 only     @_    subroutine args
  12 blcks               with EXPR      %ENV  environment