use strict;
use warnings;
use feature 'say';
use Data::Dump;
use Time::HiRes 'time';

use Digest::xxHash;

use Algorithm::Combinatorics 'combinations';

srand( 1122 );

my $len = 1000;
my $vol = 2000;

my @data = map {
    join '', map { chr rand 256 } 1 .. $len;
} 1 .. $vol;

# add some duplication

@data = ( @data,
    ( grep { rand > .8 } @data ),
    ( grep { rand > .4 } @data ),
    ( grep { rand > .2 } @data ),
);    

my %h1;
my %h2;

sub do_something {};

###########################

my $t = time;

for my $i ( 0 .. $#data ) {
    for my $j ( $i + 1 .. $#data ) {
        do_something( $i, $j ) 
            if $data[ $i ] eq $data[ $j ]
    }
}

say time - $t;

###########################

$t = time;

push @{ $h2{ $data[ $_ ]}}, $_ 
    for 0 .. $#data;

for ( grep { @$_ > 1 } values %h2 ) {
    do_something( @$_ ) 
        for combinations( $_, 2 )
}

say time - $t;

__END__

1.1641149520874
0.022386074066162