#! usr/bin/perl

use strict;
use warnings;
use diagnostics;

my $scalar_ref=*foo{SCALAR};
print "scalar ref defined\n" if defined $scalar_ref;
#------------------------#
my %hash=();
my $hash_ref=*hash{HASH};
print "hash ref defined\n" if defined $hash_ref;
__END__
Output:

Name "main::hash" used only once: possible typo at test.pl line 11 (#1)
    (W once) Typographical errors often show up as unique variable names.
    If you had a good reason for having a unique name, then just mention it
    again somehow to suppress the message.  The our declaration is
    provided for this purpose.
    
    NOTE: This warning detects symbols that have been used only once so $c, @c,
    %c, *c, &c, sub c{}, c(), and c (the filehandle or format) are considered
    the same; if a program uses $c only once but also uses any of the others it
    will not trigger this warning.
    
Name "main::foo" used only once: possible typo at test.pl line 7 (#1)
scalar ref defined