I'm not sure which kind of parameter checking you want, maybe counting a hash slice is just enough?

@allowed_keys = qw/name user host command/; 
my %allowed;
@allowed{@allowed_keys} = delete @$cmd{@allowed_keys}
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any remaining entries you get from keys %$cmd can't be allowed, an if %allowed has less then 4 entries you might need to throw an error.

Alternatively you can preset %allowed with default values in the second line, which are overwritten if present.

Hope this helps.