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

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

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