Perl internal variables. Are next expressions same?

Question

I have two expressions:

${ \'main::\' }{ \"_<$filename\" }

and

${ \"::_<$filename\" }

are these tw

Answer 1

Almost

${ "::_<$filename" } is the scalar variable in the default/main namespace with the name given by "_<$filename"

%{"main::"} is the stash variable for the default/main namespace, and ${"main::"}{"_<$filename"} is a typeglob which can be derefenced with any of the dereferencing operators to retrieve the values in the main namespace with the name given by "_<$filename".

So it's these two expressions are equivalent:

${"::_<$filename"}
${${"main::"}{"_<$filename"}}

A short demo:

$ perl -de 1

Loading DB routines from perl5db.pl version 1.37
Editor support available.

Enter h or 'h h' for help, or 'man perldebug' for more help.

main::(-e:1):   1
  DB<1> $main::foo = 42                                                                                    

  DB<2> @main::foo = (1,2,3,"bacon")                                                                       

  DB<3> x ${"main::"}{"foo"}                                                                               
0  *main::foo
  DB<4> x ${${"main::"}{"foo"}}, @{${"main::"}{"foo"}}                                                     
0  42
1  1
2  2
3  3
4  'bacon' 
  DB<5> x @{"::foo"}, ${"::foo"}                                                                           
0  1
1  2
2  3
3  'bacon'
4  42

Answer 2

No, but the following are equivalent:

${"_<$filename"}         # When this expression is found in package main
${"::_<$filename"}
${"main::_<$filename"}

${ $::{"_<$filename"} }
${ ${'::'}{"_<$filename"} }

${ $main::{"_<$filename"} }
${ ${'main::'}{"_<$filename"} }

These are refer to the variable in the root/main namespace whose name is produced by "_<$filename".

---

As a clearer example, the following are all equivalent (assuming the code is compiled in package Foo::Bar for the first two):

@moo                                  # Via hardcoded, unqualified name
@{'moo'}                              # Via symref using unqualified name

@Foo::Bar::moo                        # Via hardcoded, qualified name
@{'Foo::Bar::moo'}                    # Via symref using qualified name

@{ *Foo::Bar::moo }                   # Via hardcoded glob
@{ *{'Foo::Bar::moo'} }               # Via symref to glob

@{ $Foo::Bar::{moo} }                 # Via glob via hardcoded package
@{ ${'Foo::Bar::'}{moo} }             # Via glob via symref to package

@{ $Foo::{'Bar::'}{moo} }             # Via glob via hardcoded parent package
@{ ${'Foo'}::{'Bar::'}{moo} }         # Via glob via symref to parent package

@{ $::{'Foo::'}{'Bar::'}{moo} }       # Via glob via root package
@{ $main::{'Foo::'}{'Bar::'}{moo} }   # Same

---

Perl allows symbol names to be used as references.

$ perl -e'%h = ( a => 4, b => 5 ); my $ref = "h"; CORE::say $ref->{a};'
4

These are called "symbolic references" (and they're not allowed when using use strict;). The following uses main:: as a symbolic reference:

${ 'main::' }{ "_<$filename" }

There's no point to doing that, however. Since the name of the variable is hardcoded anyway, we might as well use the following:

$main::{ "_<$filename" }

Not only is this simpler, it's allowed when using use strict;.

---

But what is %main::? Well, that's the symbol table for the package main. The keys of %main:: are the names of the symbols (variables) that exist in the package main (without any sigil).

The values of %main:: are symbol table entries. We call these "typeglobs", or "globs" for short. Dereferencing a glob as if it was a reference access the variable of the type appropriate for the dereference. For example,

$main::{a}

returns

*main::a       # A glob

and

${ $main::{a} }

returns

$main::a

This means that the following returns a glob to the variable in the main namespace whose name is produced by "_<$filename"

$main::{ "_<$filename" }

---

The main namespace is also the root namespace. This means that

$main::a

and

$::a

both refer to the same variable. It also means that

%main::

and

%::

both refer to the same variable. It also means that

$main::{ "_<$filename" }

and

$::{ "_<$filename" }

both return the same glob.

---

We've established that the following returns a glob to the symbol in the root/main namespace whose name is produced by "_<$filename"

$::{ "_<$filename" }

We've also established that the following references the scalar in the root/main whose name is produced by "_<$filename":

${ $::{ "_<$filename" } }

And finally, we've also established that following is a symbolic reference that references the same scalar:

${ "::_<$filename" }