The other answers have given you some good background information, but just to fill you in with what you need:

sub return_ref {
    return if rand < 0.3;
    my %hash;
    $hash{foo} = 'bar' if rand < 0.5;
    return \%hash;
}

my $ref = return_ref();
if (ref($ref) eq 'HASH' and %$ref) {
    print "We have a non-empty hash reference.\n";
}
else {
    print "No hash reference, or empty hashref.\n";
}
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Run that a couple of times and observe the behaviour.