Re^2: The error says the value is uninitialized, but it works anyway

undef ne $string if $string is a defined value.

Sorry, but no: $string = "" is a defined value, and undef eq "". What's happening is that eq forces string context on its arguments and undef is converted to "", with a warning, which of course is the same thing as the string "". eq is not a test for definedness, only defined does that - undef eq undef not because they are both undef, but because "" eq "".

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Re^3: The error says the value is uninitialized, but it works anyway
by jcb (Parson) on Aug 18, 2019 at 22:30 UTC

Hmmm... You are right. I have edited my earlier post to correct the misinformation.

It seems that I had confused Perl, SQL, and Lisp while writing that. As I recall, there is only one undef value in perl, and DBI (see also DBI recipes) maps SQL NULL and Perl undef. Here are the tests I ran before writing that post:

$ perl -we 'print "T\n" if "foo" eq "foo"'
T

$ perl -we 'print "T\n" if undef eq "foo"'
Use of uninitialized value in string eq at -e line 1.

$ perl -we 'print "T\n" if undef eq undef'
Use of uninitialized value in string eq at -e line 1.
Use of uninitialized value in string eq at -e line 1.
T

Spotted the missing test yet? Try:

$ perl -we 'print "T\n" if undef eq ""'
Use of uninitialized value in string eq at -e line 1.
T

Since the message specifically mentions use of undef in eq, I had assumed that eq actually sees the undefined value, instead of stringifying it like any other value. Wrong!

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