#This bit works
foreach (@{$row_data{groups}}){ push @{$new_row_data{groups}}, $_ if ${$_}{whatever}==$something_else;

foreach (@{$row_data{fields}}){ push @{$new_row_data{fields}}, $_ if ${$_}{whatever}==$something_else;

#This bit doesn't at all
$row_data{groups}=$new_row_data{groups};
$row_data{fields}=$new_row_data{fields};