As the array size grows, it doesn't take long for the OPs original to out pace variation1. It only requires a 200,000 or so for that to happen, and the benefits mount geometrically as the array size grows:

#!/usr/bin/env perl
use strict;
use warnings;
use Benchmark qw(:all);

our @array = 'aaaa' .. 'lzzz';
print "$#array\n";

sub original {
  my %hash;
  for (my $idx=0; $idx<@array; $idx++) { $hash{$array[$idx]} = $idx;}
}

sub variation1 {
  my %hash;
  @hash{ @array } = 0 .. $#array;
}

sub variation2 {
  my %hash = map { $array[$_] => $_ } 0..$#array;
}

sub variation3 {
  my $idx = 0;
  my %hash = map { $_ => $idx++ } @array;
}

sub variation4 {
  my $idx = 0;
  my %hash; $hash{ $_ } = $idx++ for @array;
}


cmpthese -5, {
   'original'   => \&original,
   'variation1' => \&variation1,
   'variation2' => \&variation2,
   'variation3' => \&variation3,
   'variation4' => \&variation4,
};

__END__
C:\test>junk91
210911
             Rate variation2 variation3 variation1   original variatio
+n4
variation2 2.08/s         --        -2%       -36%       -38%       -4
+2%
variation3 2.12/s         2%         --       -35%       -37%       -4
+1%
variation1 3.26/s        57%        54%         --        -3%        -
+9%
original   3.37/s        62%        59%         3%         --        -
+6%
variation4 3.57/s        72%        68%         9%         6%         
+--
[download]

(I've added another variation that works better for large arrays.)