You really should consider using placeholders, as in:

#!/perl/bin/perl -w
use strict;
use Data::Dumper;

# Added for test purposes
my $lname = "foo";
my %fd = (
  txtName    => "", 
  txtCreated => "2007-04-04",
  txtDateren => "", 
  txtOldname => "Jackson",
  txtDel     => "",
);

# The actual code...
my %cols = (
  txtName    => "Listname", 
  txtCreated => "DateCreated",
  txtDateren => "DateRenamed", 
  txtOldname => "OldListname",
  txtDel     => "DateDeleted",
);

my @values = ();
my @fields = ();
for ( qw/ txtName txtCreated txtDateren txtOldname txtDel / ) {
  push (@fields, $cols{$_} . "=" . '?');
  push (@values, ($fd{$_} eq '') ? 'NULL' : $fd{$_});
}

my $updatestring = join(', ', @fields);
my $SQL = qq(
UPDATE table 
SET $updatestring 
WHERE tbl.field = '$lname'
);

print $SQL;

my $sth = $db->prepare($SQL);
$sth->execute(@values);

__OUTPUT__

UPDATE table
SET Listname=?, DateCreated=?, DateRenamed=?, OldListname=?, DateDelet
+ed=?
WHERE tbl.field = 'foo'
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