while (my $ar = $sth->fetch()) {
   my $item = $ar->[0];
   ...
}

##</code><code>##

sub make_sth_iterator {
   my ($sth) = @_;
   return sub {
      my $ar = $sth->fetch();
      return $ar ? $ar->[0] : ();
   };
}

my $iter = make_sth_iter($sth);

# Parens around $item required. Without,
# them an $item with value "" or "0"
# will exit the loop prematurely.
while (my ($item) = $iter->()) {
   ...
}