And if it's not a frequent use case, please use two lines.

The next maintainer will be thankful to easily read how @v is defined:

my @v= keys %h;
die "Got extra Z: @v\n" if @v >1
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Otherwise if you really need it that often, why don't you simply define a helper function?

   
  DB<100> sub check { die "Got extra Z: @_\n" if @_ >1 }

  DB<101> %h=(a=>1)
 => ("a", 1)

  DB<102> check my @v= keys %h
 => ""

  DB<103> %h=(a=>1,b=>2)
 => ("a", 1, "b", 2)

  DB<104> check my @v= keys %h
Got extra Z: a b
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And if you need more flexibility in testing, use some functional magic

  DB<106> sub avoid (&;@) { 
           my $code=shift;
           my $msg = $code->(@_);
           die $msg if $msg;
           }

  DB<107> avoid { "Got extra Z: @_\n" if @_ >1 } my @v = keys %h;
Got extra Z: a b

  DB<108> %h=(a=>1)
 => ("a", 1)

  DB<109> avoid { "Got extra Z: @_\n" if @_ >1 } my @v = keys %h;

  DB<110>
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but again, for the sake of readability please use a line break (even if it's a one liner)

avoid { "Got extra Z: @_\n" if @_ >1 } 
    my @v = keys %h;
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And for completeness , I'm sure you could also use variable :attributes for such checks.

 my @v :check = keys %h;

See Attribute::Handlers for details.

¹)

       The declared variable is not introduced (is not visible) until 
+after
       the current statement.  Thus,

           my $x = $x;

       can be used to initialize a new $x with the value of the old $x
+, and
       the expression

           my $x = 123 and $x == 123

       is false unless the old $x happened to have the value 123.
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