Erm ... CONVERT is a SQL function, not a Perl one, likewise the money is a SQL constant, not a Perl one. And is you enclose a variable in singlequotes you do not get the value of the variable inserted into the string, but the dollar and the name of the variable. I believe you meant:

$sql_net="UPDATE today_records
        SET current_prem = CONVERT(money, ?)
        WHERE  account_num= ? AND type='OK'";
my $sth2 = $dbh_2->prepare($sql_net);

...

$sth2->execute( $ABC7, $ABC2) || die $sth2->errstr;
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Now, while it might make sense to name the statement handle $sth2, it definitely does not make sense to name variables $ABC1, $ABC2, ... It should be an array!

BTW, are you sure the CONVERT() is needed?