Well that could be an expensive proposition. If you can live with the memory cost, something like the following will work. In this version, I'm presuming that the "collection of rows" starts out empty, and that you don't need it for anything other than to determine whether a row has been seen before or not. If that presumption is false, let me know and I can enhance it accordingly.

my %seen;
my @row;
while ( @row = $sth_C->fetchrow_array )
{
    my $krow = join "\0", @row;
    $seen{$krow}++ and next; # or did you mean last?
    print OUTPUT_FILE join( "\t", @row ), "\n";
    undef @row;
}
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