my @keeps = qw{a b};
    my %to_keep = map { $_ => 1 } @keeps;
...
        delete @$sms_ref{ grep { ! exists $to_keep{$_} } keys %$sms_ref };

##</code><code>##

...
        delete @$sms_ref{ grep { ! $to_keep{$_} } keys %$sms_ref };

##</code><code>##

    my %to_keep;
    @to_keep{ qw{a b} } = ();
...