I wasn't saying that defined() doesn't have important uses, only that it use as given in the referenced post was not useful.

That said, I wouldn't use a regex as you have either. You point out one reason yourself, though m//s would probably address that concern, but, as others pointed out earlier, using a regex to test a string against a single, predefined char (or word)--especially when that char is a non-word char, meaning that the m//i option would be of no benefit--is simply overkill.

Personally, I would probably code the test as:

if ( !defined($foo) or  $foo eq '' or $foo eq '?' ) { 
    print "\$foo OK!\n"; 
}
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as using a regex against a fixed string seems pointless and I prefer positive conditions to negative ones.

Update:Modified condition, struck irrelevant comments. Seems I lost track of the original questioners reqs. Personally, I probably still wouldn't use a regex for this.