#!/usr/bin/perl
use warnings;
use strict;

use Test::More tests => 1;

my $ranksUsed = [ [ 1 ], [ 2 ] ];

sub original {
    my @ranksUsed;
    for (@$ranksUsed) {
        push @ranksUsed, $_->[0];
    }

    my %ranksUsed = map {$_ => undef} @ranksUsed;
    return \%ranksUsed
}

sub slice {
    my %ranksUsed;
    undef @ranksUsed{ map @$_, @$ranksUsed };
    return \%ranksUsed
}

is_deeply(original(), slice(), 'same');