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 %_ listed in perlvar.

I'd normally be writing the above by declaring %seen e.g.:

my %seen; my @list = grep { not $seen{$_}++ } @list; 

But %_ seems to work, although it seems to be global scope. Can anyone point me to a reference for it? (Or at least reassure me that doing the above isn't smashing something important!)

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.