Some old PM nodes I remember that seem relevant:

• "Secret" operators
• Secret Perl Operators: the boolean list squash operator, x!!
• Flipin good, or a total flop?

From The Lighter Side of Perl Culture (Part IV): Golf, some "hidden golfing features":

Golfing Technique                     Inventor           Year
-----------------                     --------           ----
@{[]}  aka ???                        The Larry or       1994
                                      The Schwartz
}{     aka eskimo greeting            The Abigail        late 1990s
~~     aka inchworm                   ???
~-     aka inchworm-on-a-stick        The Hospel         2002
$_ x= boolean expression              The Larry          early 1990s
y///c aka Abigail's Length Horror     The Hall           1996
stuff value into $\ for printing      The van der Pijll  2001
}for(...){ variation of eskimo        The Hospel         2001
--$| magical flip-flop                The Hospel         2002
\$h{X} is one less than ++$h{X}
       aka Thelen's Device            The Thelen         2002
-i and $^I for data value             The Sperling       2002
[download]

Update: BooK has recently popularized a new set of secret operators, the screwdriver operators:

-=! and -=!!        - flathead
+=! and +=!!        - phillips
*=! and *=!!        - torx

x=! and x=!!        - pozidriv (http://en.wikipedia.org/wiki/Pozidriv)

This is a conditional "set to empty string" operator
(the string equivalent of the torx):

$x x=!! $y  is same as $x = '' unless $y;
$x x=!  $y       --    $x = '' if $y;
[download]

March 2012 Update: BooK at it again, this time proposing a new sperm secret operator. An alternative name is the "kite" secret operator.

From BooK "perlsecret - Perl secret operators and constants" draft man page:

Perl secret operators:

Operator     Nickname                Function
================================================
0+           Venus                   numification
@{[ ]}       Babycart                list interpolation
!!           Bang bang               boolean conversion
}{           Eskimo greeting         END block for one-liners
~~           Inchworm                scalar
~-           Inchworm on a stick     high-precedence decrement
-~           Inchworm on a stick     high-precedence increment
-+-          Space station           high-precedence numification
=( )=        Goatse                  scalar / list context
=< >=~       Flaming X-Wing          match input and assign captures
~~<>         Sperm
<<m=~>> m ;  Ornate double-bladed sword
-=!   -=!!   Flathead
+=!   +=!!   Phillips
x=!   x=!!   Pozidriv
*=!   *=!!   Torx
[download]

Perl secret constants:

Constant     Nickname                Value
=================================================
<=><=><=>    Space fleet             0
<~>          Amphisbaena             $ENV{HOME}
[download]

See also BooK proposes a new Perl secret operator.