perl -Mstrict -Mwarnings -e '
    use Data::Dumper;
    my @keeps = qw{a b};
    my %to_keep = map { $_ => 1 } @keeps;
    my $data = {
        count => 5509,
        sms => [{a => 1, b => 2, c => 3, d=>4}, {b => 2, c => 3, e => 5}]
    };
    print Dumper $data;
    for my $sms_ref (@{$data->{sms}}) {
        delete @$sms_ref{ grep { ! $to_keep{$_} } keys %$sms_ref };
    }
    print Dumper $data;
'
$VAR1 = {
          'count' => 5509,
          'sms' => [
                     {
                       'c' => 3,
                       'a' => 1,
                       'b' => 2,
                       'd' => 4
                     },
                     {
                       'e' => 5,
                       'c' => 3,
                       'b' => 2
                     }
                   ]
        };
$VAR1 = {
          'count' => 5509,
          'sms' => [
                     {
                       'a' => 1,
                       'b' => 2
                     },
                     {
                       'b' => 2
                     }
                   ]
        };