I was presuming the OP would replace the calls to deleteref with just the undef. You're right, of course, that making a copy of the reference in a subroutine and undeffing that is a no-op.

use Test::More tests => 2;

sub deleteref {
    my $h = shift;
    undef $h;
}

my $c = {d => 3};

deleteref($c);
is( keys %$c, 1,
    'undef on copy of hashref in a subroutine should not clear it' );


undef $c;
is( keys %$c, 0,
    'undef on hash reference, not a copy of it in a subroutine, should
+ clear it' );
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