I don’t know what you mean by a “virtual variable”, but why not just use hashes:

my %sql_count_A_Ref_vs_Comp;
my %sth_Count_Ref_vs_Comp;

for (qw( A B C D ))
{
    $sth_Count_Ref_vs_Comp{$_} = $dbh_B->prepare($sql_count_A_Ref_vs_C
+omp{$_}) or die "Cannot prepare: " . $dbh_B->errstr();
    $sth_Count_Ref_vs_Comp{$_}->execute() or die "$sth_Count_Ref_vs_Co
+mp{$_}->errstr\n";
}
[download]

Adjust as needed to accommodate $letter_tag as the hash key.

Hope that helps,