You can use a hash slice to get the same result:

#!/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');
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If it's possible for the list to be empty, you need to check for that.