Is there any situation where the 'old' syntax is preferable or even required?

It is not required for hashrefs like this but obviously you can't use the normal arrow to dereference things where there are no components. So, if you have a scalar reference the only ways to dereference that are with a $$ or using postfix dereference. In such situations I much prefer the $$ for clarity. In fact I've just tried this example to illustrate and it won't even compile with the postfix deref line uncommented (error is $* is no longer supported as of Perl 5.30).

#!/usr/bin/env perl
use strict;
use warnings;
use feature 'say';

my $foo = 'some string';
my $ref = \$foo;
say "Deref with \$\$: $$ref";
# Uncomment next line for compilation error - why?
#say "Postfix deref: $ref->$*";
[download]

Perhaps someone who likes/uses postfix deref can explain where the compilation error comes from and how to fix it. As for me, I'm quite happy with $$ in these situations.

See also I use postfix dereferencing ... for various comments on how wonderful/terrible postfix dereferencing is.

G'day hippo,

"Perhaps someone who likes/uses postfix deref can explain where the compilation error comes from and how to fix it."

The problem revolves around the version of Perl being used, the feature bundle loaded, and additional features requested. Here's some examples:

v5.36 with full feature bundle

$ perl -E 'say $^V; my $x = "xyz"; my $r = \$x; say "PFDR: $r->$*";'
v5.36.0
PFDR: xyz
[download]

v5.36 with no feature bundle

$ perl -e 'print $^V; my $x = "xyz"; my $r = \$x; print "PFDR: $r->$*"
+;'
$* is no longer supported as of Perl 5.30 at -e line 1.
[download]

v5.22 with default :5.22 feature bundle

$ perl -e 'use v5.22; say $^V; my $x = "xyz"; my $r = \$x; say "PFDR: 
+$r->$*";'
$* is no longer supported as of Perl 5.30 at -e line 1.
[download]

v5.22 with default :5.22 feature bundle plus "postderef" feature

$ perl -e 'use v5.22; use feature "postderef"; say $^V; my $x = "xyz";
+ my $r = \$x; say "PFDR: $r->$*";'
$* is no longer supported as of Perl 5.30 at -e line 1.
[download]

v5.22 with default :5.22 feature bundle plus "postderef" feature (dereferencing outside of quotes)

$ perl -e 'use v5.22; use feature "postderef"; say $^V; my $x = "xyz";
+ my $r = \$x; my $y = $r->$*; say "PFDR: $y";'
v5.36.0
PFDR: xyz
[download]

v5.22 with default :5.22 feature bundle plus "postderef_qq" feature

$ perl -e 'use v5.22; use feature "postderef_qq"; say $^V; my $x = "xy
+z"; my $r = \$x; say "PFDR: $r->$*";'
v5.36.0
PFDR: xyz
[download]

v5.24 with default :5.24 feature bundle

$ perl -e 'use v5.24; say $^V; my $x = "xyz"; my $r = \$x; say "PFDR: 
+$r->$*";'
v5.36.0
PFDR: xyz
[download]

I couldn't see it specifically documented anywhere but 'use feature "postderef_qq";' seems to imply 'use feature "postderef";'. This is borne out by my own tests (in particular, see the "v5.22" tests above) and documentation of feature bundles that include postderef_qq do not also include postderef.
[I have a niggling feeling that I did see such documentation a long time ago. If someone locates it, please share. Thankyou.]

Some relevant documentation links (in no particular order):

• use VERSION
• FEATURE BUNDLES
• The 'postderef' and 'postderef_qq' features
• Postfix Dereference Syntax
• $*

— Ken

Thanks for the detailed analysis, kcott. I am using 5.34 so adding in use v5.34; has allowed the construct to work for me. use feature 'postderef_qq'; also works.

However, the version-dependent nature of all of this combined with the paucity of documentation and the arcane error message received has only served to reinforce my decision to avoid postfix dereferencing in general.

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G'day Bod,

It hadn't occurred to me when reading earlier posts referring to "old syntax", but your use of "modern syntax" made me wonder how much older $$hashref{key} was compared to $hashref->{key}.

The oldest Perl version available via the online "Perldoc Browser" is for "Perl 5.005". Poking around in various parts of that (perlop, perldsc, and others) I see both forms being used without any mention of one being newer. I'm happy to be proven wrong, but I rather suspect that both have been available since Perl 5.000.

Without the arrow (dereferencing) operator, it's easy to make mistakes, especially with complex data structures. This is discussed in "5.005: perlref: Using References"; this information is retained in "5.36.1: perlref: Using References".

Except in the simplest of cases, I'll always prefer the $hashref->{key} form, as I find it both easier to read and less error-prone. I can't think of any situation where the $$hashref{key} form is "required".

In closing, it's probably worth noting "Postfix Dereference Syntax" and "Postfix Reference Slicing". These are newer syntaxes. I prefer them; I acknowledge that others don't. There's an example of the former in my reply to the OP.

I rather suspect that both have been available since Perl 5.000

You could well be right. The "old" label is more referring to when it was in more common use. Back in the day it was everywhere and if that's what you read then that's what you write. I guess the more that I developed as a Perl programmer the more I dealt with complex data structures and the ever more unwieldy the old syntax became. No idea if that's a general thing or specific to me but it's the most likely reason I can see looking back.

Except in the simplest of cases, I'll always prefer the $hashref->{key} form, as I find it both easier to read and less error-prone. I can't think of any situation where the $$hashref{key} form is "required".

Completely agree on this.

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🦛