Recently I've encountered some code hidden in a 1000-line long file that looked like this:

examples/return_or.pl

use strict;

sub compute {
    my ($param) = @_;

    # ...
    return $param or 'default';
}

print '1: ', compute('hello'), "\n";
print '2: ', compute(''), "\n";

I am not even sure how I noticed it, but it looked incorrect so I created the above simple example.

If we run that script we get:

1: hello
2: 

I think the intention was to return the 'default' value if the $param is empty, but that should be written as:

    return $param || 'default';

or, if we only want to use the 'default' value in case $param is undef then, starting from perl 5.10 we should write

    return $param // 'default';

The problem with using or is that its precedence is lower that that of the return call.

We can use B::Deparse to learn what perl really thinks about our code.

Running

perl -MO=Deparse,-p examples/return_or.pl

We get the following output:

examples/return_or_deparsed.pl

sub compute {
    use strict;
    (my($param) = @_);
    ((return $param) or 'default');
}
use strict;
print('1: ', compute('hello'), "\n");
print('2: ', compute(''), "\n");

Look at line 4. It is of course strange, but basically it means

Return the content of $param and if the return call resulted 
something false, then check the content of 'default',
but don't do anything with it.

Not very clever, but that's what perl understands.

How to locate these issues?

Now that we know this code is incorrect, we can hopefully go and fix it replacing or with ||, but given a huge code-base, how can we find all the other places where such code was written?

As you can notice this code-base omitted the use warnings statement.

So what happens if we add use warnings; ?

Nothing.

We get the same results and no warnings.

We use perl 5.16 in our system and As it turns out this specific issue has not triggered a warning in that version of perl. Not even in 5.18.2.

The first version I have on my computer that warns about this construct is perl version 5.20.

It says:

Possible precedence issue with control flow operator.

Conclusion

Always use warnings, and even if you don't plan to upgrade to newer version of perl, run your tests there too. At least your compile-test. They can reveal all kinds of potential bugs.

Also check for no warnings during the tests. That will help you locate new warnings that were not around in previous versions of Perl.