What is “%_” in perl?

Question

I\'ve just been given a code snippet:

@list = grep { !$_{$_}++ } @list;

As an idiom for deduplication. It seems to work, but - there\'s no

Answer 1

It's a hash. You can have a hash named _ because _ is a valid name for a variable. (I'm sure you are familiar with $_ and @_.)

No Perl builtin currently sets it or reads %_ implicitly, but punctuation variables such as %_ are reserved.

Perl variable names may also be a sequence of digits or a single punctuation or control character (with the literal control character form deprecated). These names are all reserved for special uses by Perl

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Note that punctuation variables are also special in that they are "super globals". This means that unqualified %_ refers to %_ in the root package, not %_ in the current package.

$ perl -E'
   %::x    = ( name => "%::x"    );
   %::_    = ( name => "%::_"    );
   %Foo::x = ( name => "%Foo::x" );
   %Foo::_ = ( name => "%Foo::_" );

   package Foo;

   say "%::x    = $::x{name}";
   say "%::_    = $::_{name}";
   say "%Foo::x = $Foo::x{name}";
   say "%Foo::_ = $Foo::_{name}";

   say "%x      = $x{name}";
   say "%_      = $_{name}";
'
%::x    = %::x
%::_    = %::_
%Foo::x = %Foo::x
%Foo::_ = %Foo::_
%x      = %Foo::x
%_      = %::_      <-- surprise!

This means that forgetting to use local %_ (as you did) can have very far-reaching effects.