my $aref = [ 1..5 ];
bless $aref, 'HASH';

# An array ref now thinks it's a hash ref
print "aref is a HASH\n" if ref($aref) eq 'HASH';
print "aref is a HASH\n" if UNIVERSAL::isa($aref, 'HASH');

my $href = { 0..9 };
# This is ok
print "href is a hash\n" if ref($href) eq 'HASH';

bless $href, 'Some::Package';
# This may not be ok
print "href is not a hash\n" unless ref($href) eq 'HASH';