use strict;
my $sth_null = $dbh->prepare(q{
  SELECT id, name FROM category WHERE parent IS NULL
});
$sth->execute or die $dbh->errstr;

my $sth_parent = $dbh->prepare(q{
  SELECT id,name FROM category WHERE parent = ?
});

while ( my @slog = $sth_null->fetchrow_array ) {
  $sth_parent->execute($slog[0]) or die $dbh->errstr;
  my $rv = $sth_parent->rows;
  my $subs = $rv;

...