Firstly, it looks like you have a typo in the "unless" line, $k should perhaps be $key.

Secondly, using a scalar and concatenating spaces and bad keys onto a string seems a little awkward. Using an array instead and taking advantage of how it is interpolated inside double quotes seems more natural. I'm not sure why you use a mixture of single and double quotes along with concatenation in your print statement, just interpolate inside double quotes.

use strict;
use warnings;

use feature qw{ say };

my %allowed = map { $_ => 1 } qw{ name user host command };

my @commands = (
   {
      name    => q{Fred Bloggs},
      user    => q{fbloggs},
      host    => q{red},
      command => q{ls},,
   },
   {
      name     => q{Charlie Farley},
      user     => q{cfarley},
      host     => q{green},
      zone     => q{A},
      building => q{Chaucer},
      command  => q{ps},
   },
   );

for my $command ( @commands )
{
    my @badKeys = grep { not exists $allowed{ $_ } } keys %$command;
    say
       @badKeys
       ? qq{Command invalid: bad keys: @badKeys}
       :  q{Command OK};
}
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The output.

Command OK
Command invalid: bad keys: zone building
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I hope this is of interest.