$ cat test.pl
#!/usr/bin/perl
*STDERR = *STDOUT;
$|=1;

my $hr = bless {a=>1, b=>2}, "food";

print "Manual deref: ", join(", ", keys %$hr), "\n";
print "Auto deref: ", join(", ", keys $hr), "\n";

$ perl test.pl
Manual deref: a, b
Type of argument to keys on reference must be unblessed hashref or arrayref at test.pl line 8.