I tried to reproduce the problem, but I couldn't get the same result you mentioned.

#!/usr/bin/perl -w
use DBI;
use strict;

my $dbh = DBI->connect("DBI:mysql:test;host=localhost"
    . ";mysql_read_default_file=$ENV{HOME}/.my.cnf",
    undef, undef, {RaiseError => 1})
    or die "can't connect\n";

my $pairs = {
    ONE => 'RTEST',
    TWO => undef,
    THREE => 'ANOTHER'
};

# Creating a dummy table to match the example
$dbh->do(qq{CREATE TABLE IF NOT EXISTS web_sessions_aux
 (wsa_id INT not null, wsa_type char(10), wsa_key char(10),
 wsa_value char(10))});

my ($id,$type) = (1,'UNKNOWN');
my $sth = $dbh->prepare("
                      INSERT INTO web_sessions_aux
                      (wsa_id, wsa_type, wsa_key, wsa_value)
                      VALUES (?,?,?,?)");

DBI->trace(2);
foreach my $key (keys %$pairs) {
    my $key_value = defined($pairs->{$key})?
                            $pairs->{$key} : '';

     $sth->execute($id,$type,$key,$key_value);
}
DBI->trace(0);

$dbh->disconnect();
[download]

This sample script shows what is going on behind the scenes.

    DBI 1.21-nothread dispatch trace level set to 2
    -> execute for DBD::mysql::st (DBI::st=HASH(0x81342c0)~0x80b1264 1
+ 'UNKNOWN' 'TWO' '')
    -> dbd_st_execute for 081342f0
      Binding parameters: INSERT INTO web_sessions_aux
                                    (wsa_id, wsa_type, wsa_key, wsa_va
+lue)
                                    VALUES (1,'UNKNOWN','TWO','')
    <- dbd_st_execute 1 rows
    <- execute= 1 at dbi_quoting_problem.pl line 29
    -> execute for DBD::mysql::st (DBI::st=HASH(0x81342c0)~0x80b1264 1
+ 'UNKNOWN' 'THREE' 'ANOTHER')
    -> dbd_st_execute for 081342f0
      Binding parameters: INSERT INTO web_sessions_aux
                                    (wsa_id, wsa_type, wsa_key, wsa_va
+lue)
                                    VALUES (1,'UNKNOWN','THREE','ANOTH
+ER')
    <- dbd_st_execute 1 rows
    <- execute= 1 at dbi_quoting_problem.pl line 29
    -> execute for DBD::mysql::st (DBI::st=HASH(0x81342c0)~0x80b1264 1
+ 'UNKNOWN' 'ONE' 'RTEST')
    -> dbd_st_execute for 081342f0
      Binding parameters: INSERT INTO web_sessions_aux
                                    (wsa_id, wsa_type, wsa_key, wsa_va
+lue)
                                    VALUES (1,'UNKNOWN','ONE','RTEST')
    <- dbd_st_execute 1 rows
    <- execute= 1 at dbi_quoting_problem.pl line 29
[download]

Since the DBI seems to be doing its honest job correctly, I would investigate some side effects somewhere else. Is the problem replicable? Could you isolate the strange behavior in a stand-alone (possibly short) script? That would be useful to understand where the problem lays.
As an additional thought, this case looks similar to DBI-trace and $hDB-quote() interaction. The problem there was related to some CGI variables behaving differently under trace(0) and trace(2). I was not able to find out what exactly was happening in that case, except that interpolating the variable made the problem disappear.
In this case, with or without trace(2), I get a correct database update.
Maybe, if the two cases are related, we could reach a more enlightening conclusion.

HTH

 _  _ _  _  
(_|| | |(_|><
 _|
[download]

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