Does %_ have a function or is it just for fun?

larard has asked for the wisdom of the Perl Monks concerning the following question:

I was recently bitten by a bug in my code, where I accidentally wrote

 
$_{foo}; #$_ containts a hashref.
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I had meant to use $$_{foo}, but was suprised that despite using strict and warnings, perl had seemingly autodeclared %_.

My original code read something like this, to show it isn't completely contrived.

use strict;
use warnings;
use DBI;
use Data::Dumper;

#connect and execute stored procedure 
#...
push @results, $_{user_id} 
     while ($_ = $sth->fetchrow_hashref());
print Dumper(\@results);
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Which prints

$VAR1 = [
          undef,
          undef,
          undef
        ];
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I soon spotted the problem, caused by my ever shoddy typing. However a trip to perlvar failed to enlighten. Further investigation revealed that %_ does indeed exist, but is perhaps unused. 'What is %_' was asked a little over 7 years, but didn't elicit a definitive answer.

What is %_ for? If there is no current use, would anyone care to suggest what %_ might be used for?

Comment on Does %_ have a function or is it just for fun? Select or Download Code

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Re: Does %_ have a function or is it just for fun? by ikegami (Patriarch) on Jul 16, 2007 at 17:48 UTC
Punctuation variables don't trigger strict errors or "used only once" warnings, even if they have no use.	[reply] [d/l]
Re^2: Does %_ have a function or is it just for fun? by derby (Abbot) on Jul 16, 2007 at 20:21 UTC
and that's documented in the Technical Note on the Syntax of Variable Names in perlvar. -derby	[reply]
Re: Does %_ have a function or is it just for fun? by injunjoel (Priest) on Jul 16, 2007 at 17:55 UTC
larard, I use `%_` in my code here and there. I even mention it's use in The Uniqueness of hashes. Have a look. -InjunJoel "I do not feel obliged to believe that the same God who endowed us with sense, reason and intellect has intended us to forego their use." -Galileo	[reply] [d/l]
Re: Does %_ have a function or is it just for fun? by FunkyMonk (Bishop) on Jul 16, 2007 at 21:23 UTC
I seem to remember reading somewhere (ie I might be wrong), that due to the magic nature of typeglobs and the punctuation variables, that you can't just get a single type for each. That probably isn't too clear, it's easier with an example: Since Perl wants `$_`, you also get `%_`, `@_` and `&_` for free. My (very limited) testing demonstrates that that's the case for other punctuation variables, too: `use strict; use warnings; use Data::Dumper; # $\ - output record separator %\ = ( 1 => 2); @\ = ( 1, 2 ); print Dumper \%\, \@\;; # $@ - eval error %@ = ( 3 => 4); @@ = ( 3, 4 ); print Dumper \%@, \@@;` [download]	[reply] [d/l] [select]
Re: Does %_ have a function or is it just for fun? by ForgotPasswordAgain (Vicar) on Jul 16, 2007 at 17:37 UTC
I don't know "why", but I just note incidentally that you can also do that with the coderef slot: `$ perl -Mstrict -Mwarnings -le'*_=sub{"hi"}; print &_' hi` [download]	[reply] [d/l]
Re^2: Does %_ have a function or is it just for fun? by ikegami (Patriarch) on Jul 16, 2007 at 17:54 UTC
There's nothing special about `_` in your post. Typeglobs are unconditionally not subject to strict. `>perl -Mstrict -Mwarnings -le"*f=sub{'hi'}; print &f" hi` [download]	[reply] [d/l] [select]
Re: Does %_ have a function or is it just for fun? by naikonta (Curate) on Jul 17, 2007 at 06:51 UTC
What is %_ for? Since it's not documented in perlvar, I don't think it's for particular use. I think that FunkyMonk's reply is making sense. I also got the same impression upon reading about symbol table in perlmod and perlref. If there is no current use, would anyone care to suggest what `%_` might be used for? Well, you can use it as you will normally use hash. Initialize it, assign elements to it, get a value from a key. Since it's sort, it's also perfect for golf. And it can also be confusing so one may use it in obfucasted code. You can use it to alias a hash variable with some lengthy you'd be tried to type it over and over. `my %hash_variable_with_not_actually_that_long_name = (one => 1); print $hash_variable_with_not_actually_that_long_name{one}, "\n"; _ = \%hash_variable_with_not_actually_that_long_name; print $_{one}, "\n"; # output 1 1` [download] Additionally, with the aliasing, you can alias other sigils at once. But, by using `_` you're likely to get into problem. `$_ = 2; print $_, "\n"; _ = hash_variable_with_no_actually_that_long_name; print $_{one}, "\n"; print $_, "\n"; # output 2 1 (empty, with 'uninit' warnings under -w)` [download] Open source softwares? Share and enjoy. Make profit from them if you can. Yet, share and enjoy!*	[reply] [d/l] [select]
Re: Does %_ have a function or is it just for fun? by impossiblerobot (Deacon) on Jul 18, 2007 at 12:42 UTC
Variables/symbols that start with punctuation (including punctuation variables themselves) are automatically global, and forced into main:: ... except if the punctuation it starts with is the underscore, which doesn't make it automatically global ... except if it is the underscore variable/symbol itself, which is also automatically global. Maybe I need to rewrite that. It's even confusing me now. Impossible Robot	[reply]