#! 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 nam
+es.
    If you had a good reason for having a unique name, then just menti
+on 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 s
+o $c, @c,
    %c, *c, &c, sub c{}, c(), and c (the filehandle or format) are con
+sidered
    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
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