You need to change Perl-false values to undef.

From DBI:

NULL Values

Undefined values, or undef, are used to indicate NULL values. You can insert and update columns with a NULL value as you would a non-NULL value. These examples insert and update the column age with a NULL value:

  $sth = $dbh->prepare(qq{
    INSERT INTO people (fullname, age) VALUES (?, ?)
  });
  $sth->execute("Joe Bloggs", undef);
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Later

What I failed to say was that the first sentence quoted above would have been better written as:

Undefined values, or undef, are THE ONLY WAY used to indicate NULL values.

SQL, for reasons better understood by by its creators than me, makes a huge distinction between NULL values and either empty strings or zero values. Because of this, a WHERE clause that needs to account for both has to jump through hoops like

WHERE ISNULL(age, xx) = ISNULL(?, xx)
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or

$where_clause = defined $age? "age = ?" : "age IS NULL";
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My guess is that due to that NULL vs '' distinction the DBI module intentionally does not treat '' or 0 as NULL.