defined $var or equivalently defined($var) will return the integer 1 (which is a TRUE value; 1 is also TRUE in C, so this shouldn't confuse you) if the variable is defined. It will return undef (which is a FALSE value) a FALSE value (see haukex's answer) if the variable is undefined. You then take that value, either 1 or undefthe FALSE value, and stringify it. The integer 1 stringifies into "1". The FALSE value undef stringifies into "". If you don't want undef FALSE to become "", don't stringify. (The eq operator is forcing the stringification on both its arguments.)

If you really just want a boolean that decides whether the $var is defined or not, just use the truthiness of the result of defined $var -- that is explicitly the boolean test for whether the $var is defined, and the defined $var and !defined $var syntax are explicitly saying "variable is defined" and "variable is not defined". This is similar to C: if you define a function int is_five(int x) { return (x==5); }, then the return value of is_five(var) and !is_five(var) are explicit ways of testing whether or not the variable is 5. From your claim, in C, I would have to write is_five(var)==-1 to verify that var is 5, and is_five(var)==0 to verify that var is not 5, which I vehemently disagree with: that notation obfuscates what c is doing, not clarifies what it's doing internally. Just trust that Perl will do the right thing with boolean expressions in a boolean context, just like you trust that C does the right thing with boolean results in a boolean context.

if it's the lack of parentheses that are confusing you, then use the parentheses.

Aside: Urgh... I did one last refresh before hitting create, and saw that haukex beat me by a minute or two again. :-(. I went to all the trouble of writing this up, so I'll hit create anyway.

update: I was wrong: defined($var) doesn't return undef or 1; it returns the special value, as haukex said. 

c:> perl -le "print defined($x)//'<undef>'; print defined($x)||'untrue
+'"

untrue

c:>

[download]

In reply to Re^3: Regex result being defined when it shouldn't be(?) by pryrt
in thread Regex result being defined when it shouldn't be(?) by chenhonkhonk

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