I don't think you should quote a variable when using placeholders. Try

$scode ="1640039";
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in your placeholder section.

Using placeholders is a recommended best practice, but if you insist on not using placeholders, you should also use the quote operator, rather than quote yourself. I assume the KIHON1 column is of type VARCHAR2. You don't need to quote if it's a NUMBER column.

my $scode = q{1640039};
my $sql = q{ SELECT KIHON3 FROM CBMSCP.VIEWCTC_FORMI1 WHERE KIHON1 = }
+ . $db->quote($scode);
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