Perl constant hashrefs are actually functions 

To get the keys of a constant hashref you first need to learn that a constant is actually a function and hence use () to get the keys from it


INPUT:
$ cat perlConstant.pl 
use Data::Dumper;
use constant FOOHASH => {     "foo" => "bar",  };
print FOOHASH->{"foo"}."\n";
foreach $key (keys %{FOOHASH()})
 {      FOOHASH->{$key} = "bar1";  }
print FOOHASH->{"foo"}."\n";
foreach $key (keys %{FOOHASH})
 {      FOOHASH->{$key} = "bar2";  }
print FOOHASH->{"foo"}."\n";


OUTPUT:
$ perl perlConstant.pl
   bar
   bar1
   bar1

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