I think you can minimize multi-opening files by opening once:

    open FH, "+>>$virttmp" or die "Cannot open file: $!\n";
    seek FH, 0, 0;
    my @v = <FH>;
    seek FH, 0, 0;
    truncate $virttmp, 0;
    print FH sort @v;
    close FH;
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