Apart from anything else, you're re-preparing the update statement every time around the main loop, despite its text not changing. This is exactly why bind variables (which you're using) were introduced.

print "Selecting.... \n";
$sel=$dbh->prepare("SELECT name,address_number
                   from many_address      
                   WHERE name=?
                   AND number=?
                   AND street=? ");
$upd=$dbh->prepare("UPDATE many_address
                    SET name=NULL,
                    date_loaded=NULL
                    WHERE name=?
                    AND number=?
                    AND street=?");
while ($_=<PA>) {
      ($name,$add,$street)=split(/\|/,$_);
      chomp ($name,$add,$street);
      $name = '' unless defined $name;
      $sth->execute($name,$add,$street);
      #$rec=$sth->dump_results(); ### see results
      @rec = $sth->fetchrow_array();
      if (@rec) { ### check to see if @res is valued
         print "$name $add,$street --> @rec. \n";
         $upd->execute($name,$add,$street);
         print "Clearing $name,$add,$street -->@rec \n";
      } else {
         print "Not available --> \@rec is empty\n";
      }
}
close(PA);
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--
Tommy
Too stupid to live.
Too stubborn to die.