I think defined() is a good idea if there's a possibility of $foo being undefined, so as to avoid a warning about an undef being used in a string comparison/regex. Assuming an undefined value counts as a blank, I would write it like this:

if (!defined($foo) or $foo =~ m/^\??$/) {
    # Foo is valid
}
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Thus: "If foo is undefined, or matches an empty string or a single question mark."

But wasn't there something about \n's counting as "^" and/or "$"? Hmmm...

The Secret to Fortune Cookies in One Line
print join("... in bed", `fortune fortunes` =~ m/^(.*)(\.|\?|\!)$/), "\n";

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.

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What's this about a "crooked mitre"? I'm good at woodwork!

I havent read the rest of the thread, I only comment because of your mistake in the cb ;-), but the test you describe:

if ( defined($foo) and $foo eq '?' ) { 
    print "\$foo OK!\n"; 
}
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Would be better (Lazier) written

print "\$foo OK!\n" 
   if $foo and $foo eq '?';
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in my opinion anyway...

Note that since $foo must be a '?', then it cant be _any_ false value, defined being only one of three. Thus you can dispose of the defined test and replace it with simple true or false.

Yves / DeMerphq
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Software Engineering is Programming when you can't. -- E. W. Dijkstra (RIP)