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my @v= keys %h;
die "Got extra Z: @v\n" if @v >1

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

...

  DB<104> check my @v= keys %h
Got extra Z: a b

  DB<106> sub avoid (&;@) { 
           my $code=shift;
...
  DB<109> avoid { "Got extra Z: @_\n" if @_ >1 } my @v = keys %h;

  DB<110>

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

       The declared variable is not introduced (is not visible) until 
+after
       the current statement.  Thus,
...
           my $x = 123 and $x == 123

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