Some old PM nodes I remember that seem relevant:

• "Secret" operators
• Secret Perl Operators: the boolean list squash operator, x!! (and Re^2: Secret Perl Operators: The Highlander List Asserter)
• 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 baby cart                  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
[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 perlsecret:

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 aka Saturn       scalar / list context
=< >=~       Flaming X-Wing          match input and assign captures
~~<>         Sperm
<<m=~>> m ;  Ornate double-bladed sword
-=!   -=!!   Flathead
+=!   +=!!   Phillips
x=!   x=!!   Pozidriv
*=!   *=!!   Torx
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Perl secret constants:

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

October 2014 Update: Re: Perl Idioms Explained - ${\$obj->method} and @{[sort @list]} by ambrus proposes a new "twin baby cart" operator, as used by tybalt89 and tails in winning golf entry at shinh's golf site:

/[3-9]/||print"@{[world,hello,'world!']}[/./g]
"for 0..$$
[download]

See also: BooK proposes a new Perl secret operator (Secret Operator References)

I think this series is related as well:

• Perl Idioms Explained - $|++
• Perl Idioms Explained - keys %{{map{$_=>1}@list}}
• Perl Idioms Explained - ${\$obj->method} and @{[sort @list]}
• Perl Idioms Explained - my $string = do { local $/; <FILEHANDLE> };
• Perl Idioms Explained - @ary = $str =~ m/(stuff)/g
• Perl Idioms Explained - && and || "Short Circuit" operators
• Perl Idioms Explained - my ($foo, $bar) = @{shift(@_)}{qw/ -foo -bar /}
• Perl Idioms Explained - !!expr
• Perl Idioms Explained - my $count = () = /.../g

I believe that @{[]} is known as the "baby buggy". Can't remember where I read that though.

Originally, a crude name was proposed for this operator, as indicated by this response from BooK to Sebastien Aperghis-Tramoni:

> Hey Philippe, why don't you give the name we found for @{[]} ?

Because *you* found it, and want to have *my* name associated with it.

To deflect attention from the taboo name, BooK started another fwp thread, suggesting "Baby Cart" ... which was objected to by native English speakers on the grounds they'd never heard it used in everyday speech. Suggested alternatives included "Baby Carriage" and "Pram". Though many other names have been proposed, including:

• Supermarket Trolley
• Luge or Sled
• Fat Brackets
• Pram
• Turtle (update: suggested by Peter Krumins)
• Code Interpolation (update 2021: suggested by the LanX)
• Mexican Sledge (update 2021: suggested by the LanX ... which I sledged :)
• Mexican Sleigh (update 2021: LanX wearing a sombrero, riding a sleigh)

it seems that BooK's quirky Baby Cart has won this fascinating naming war.

BooK References

• Re^7: Ways of quoting (where BooK explains why he chose the Baby Cart name)
• Re^2: Splitting Hairs [tangent to: Splitting in while loop]
• Interview with Philippe Bruhat (August 2015)

Quoting BooK from Interview with Philippe Bruhat (August 2015):

I've been hanging around on the Fun with Perl mailing for a long time, and this is where José Castro started to ask for the names of the most famous secret operators for his OGSOP (Obfuscation, Golfing and Secret Operators in Perl) book project. I came up with the name "baby cart" for @{[]}, and I loved that name so much that I wanted to make sure it would be the one that everyone would use. The only way to do that was to write the reference manual myself...

Early Historical References

• Re: Hidden features of Perl (More Secret Operator References) (Baby cart operator was not "designed" by Larry when he designed Perl, rather it was independently invented by Larry and Randal shortly after Perl 5 was released in 1994 - though it did not have a name back then)
• Re^3: meaning of =()= (where merlyn rues not asking Larry Wall's daughter Heidi to coin a name back in 1994 after Heidi had successfully coined the name for the Spaceship operator <=>)
• "Some Standard Golfing Techniques" section at The Lighter Side of Perl Culture (Part IV): Golf

Comparing Baby Cart with String Interpolation in Other Languages (Golf)

Though usually too long for golf, Baby Cart occasionally features in competitive golf solutions, as you will discover by searching for Baby Cart in the following nodes:

• The golf course looks great, my swing feels good, I like my chances (Part VI)
• Spending Time on Saving Time [golf]

Based on these nodes, consider how to create a "Dear John" string in each of the four languages:

"Dear $name"              # Perl and PHP
"Dear %s" % expr          # Python and Ruby % printf-like operator
"Dear {0}".format(expr)   # Python format string method
"Dear "+`expr`            # Python backticks (TMTOWTDI)
"Dear #{expr}"            # Ruby string interpolation
"Dear @{[expr]}"          # Perl "Baby Cart" string interpolation
"Dear {expr}"             # Perl 6 version of Baby Cart (I think)
[download]

My 102 stroke Perl solution to the Saving Time challenge used Baby Cart to create a printf format string on the fly:

printf"%@{[.1*vec'XXXXXXXXXXXX',$_,8]}s",($_^$`%12?g:p)&($_^$'/5?g:u)|
+"H

"for map{$_,11-$_}<>!~/:/..5
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What's the shortest way to create this peculiar printf format string from an expression (expr) in each of the four languages?

Technique	Perl	Ruby	Python	PHP
String interpolate	"%@{[expr]}s"	"%#{expr}s"	"%%%ds"%expr	~X.expr.s
Number of strokes	13	11	12	9

where X above is the character with ord value 218. Poor old Baby Cart baby-stepped to the finish line in last place in this oddball race. This is hardly surprising given Baby Cart was not really designed; it was independently "invented" shortly after Perl 5 was released in 1994 by both L.Wall and R.Schwartz ... and later celebrated as one of Perl's secret operators.

Updated: Many changes were made and extra references added long after the original response was made.

Tadpole secret operator ~~($$..!$$) can be used as a kind of counter

print $_ , ' => ', ~~($$..!$$), "\n"
    for qw(Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec);
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Esoteric? What the heck is esoteric about them? Agrrr. Anything that's not apparent to someone with strictly imperative background with no computer science knowledge whatsoever is deemed "esoteric". Nonsense.

W(hat|TF) does this have to do with "imperative" or "computer science?" It seems non-obvious to me that a magic filehandle called "DATA" reads everything in the current file after a line containing only "__DATA__". Perl has a lot of non-obvious features, and that thread lists quite a few of them.

It seems that you are arguing semantics, which isn't the point of this post.. post whatever _you_, personally, consider a "hidden feature". Don't worry about what others consider esoteric. :)

Most of the stuff is something that looks esoteric to someone who was taught C or Fortran or who taught him/her-self VB and is so used to imperative programming, that he/she cannot think outside that old tiny box. The people that find references hard and higher order functions rocket science.

And the DATA filehandle is no more esoteric than a print statement. A bit unusual maybe, non-obvious probably, but esoteric? That's a very big stretch of the meaning of the word.

Jenda
Enoch was right!
Enjoy the last years of Rome.

s|hidden|little-known but useful|g
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