I'm writing a sub that checks values of a hash, %foo
I'm only interested in seeing existing values, not creating new ones, so the sub works something like this : 
sub check_foo {
if (exists $foo{bar}) 
    {return $foo{bar}} 
else 
    {return $the_mysterious_errorcode} 
}

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my problem is that $foo{bar} may contain undef as a value, as well as a large array of text & etc.. What can $the_mysterious_errorcode be set to so that I can differentiate between a nonexistant $foo{bar} and one that's undef?
Is it acceptable to set $! in a situation like this?

In reply to detecting errors when a sub returns undef by boo_radley

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