Further down in this thread, I got trolled by an Anon Monk.

Instead of complicated syntax, I would recommend something easier.
Performance-wise it is always better to return fewer things from the sub() that more things.
That should be obvious!

The fastest way (performance-wise) and often the easiest way for a sub() to return a hash to the caller is via a reference to the hash.
Don't worry at all about taking a couple of lines of code vs one line. Sometimes 3 lines are faster and easier to understand than a single line.

Here are some benchmarks for your perusal.
Number (3) below is a "go to" winner!

use strict;
use warnings;
use Benchmark;
use Data::Dumper;

# Note: You can increment a string in Perl! WOW!
# As long as you don't use the string in a numeric context!
# $string++ is quite different than $string +=1
#
# Below, this is used to make a bunch of unique hash keys
# I don't think that the fact that they are sequential in
# an alphabetic sense makes much difference in the generated
# hash table because of the way that the Perl hash algorithm
# works.


sub return_hash
{
   # create a large hash and return that entire hash as list
   
   my $string = "ABCDEFGHIJ";
   my %hash = map{$string++ => 1}(1..100000);
   return %hash;
} 

sub return_hash_ref
{
   # create a large hash and return a ref to that hash
   
   my $string = "ABCDEFGHIJ";
   my %hash = map{$string++ => 1}(1..100000);
   return \%hash;
}

sub return_just_keys
{
   # create a large hash and return just the keys of that hash
      
   my $string = "ABCDEFGHIJ";
   my %hash = map{$string++ => 1}(1..100000);
   return keys %hash;
}

timethese(1000, {
    '1)Keys of Hash via list' => 'my @keys = keys %{{return_hash()}}',
    '2)Keys of Local Hash copy' => 'my %hash2 = return_hash(); my @key
+s = keys %hash2;',
    '3)Keys of local Hash Ref' => 'my $href = return_hash_ref(); my @k
+eys = keys %$href;',
    '4)Just the returned keys' => 'my @keys = return_just_keys()',
    
});

__END__

Benchmark: timing 1000 iterations of 
   1)Keys of Hash via list, 
   2)Keys of Local Hash copy, 
   3)Keys of local Hash Ref, 
   4)Just the returned keys...
   
1)Keys of Hash via list: 182 wallclock secs (179.20 usr +  1.97 sys = 
+181.17 CPU) @  5.52/s (n=1000)
2)Keys of Local Hash copy: 184 wallclock secs (182.69 usr +  0.42 sys 
+= 183.11 CPU) @  5.46/s (n=1000)
3)Keys of local Hash Ref: 130 wallclock secs (129.56 usr +  0.72 sys =
+ 130.28 CPU) @  7.68/s (n=1000)
4)Just the returned keys: 125 wallclock secs (125.20 usr +  0.52 sys =
+ 125.72 CPU) @  7.95/s (n=1000)
[download]

Added: When looking at benchmarks, don't worry so much about 182 vs 184. That could be an artifact of a particular run. Most of the CPU MIP's are being consumed by creating and re-creating the hash. Look at ~180 vs ~130. The relative performance difference between the 2 methods of passing the result back is actually much, more than that because generating the hash "takes a lot of effort".

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