Install Sort::External from CPAN. It was written for the purpose of sorting huge files.

    my $sortex = Sort::External->new();
    while (<HUGEFILE>) {
        $sortex->feed($_);
    }
    $sortex->finish;
    my $prev = '';
    while ( defined( $_ = $sortex->fetch ) ) {
        next if $_ eq $prev;
        print OUTFILE $_;
        $prev = $_;
    }
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