What is wrong in this Perl expression?

Question

What's the problem with following. I am getting $attribute not defined error.

if (my $attribute = $Data->{'is_new'} and $attribute eq 'Y') {
}

Answer 1

You're being too clever. Just do this:

my $attribute = $Data->{'is_new'};

if (defined $attribute && $attribute eq 'Y') { ... }

The problems are twofold:

• You have an extra ) in your if
• my in expression context binds very tightly; $attribute is not in lexical scope until the body of the conditional statement that contains it, so the other branch of the and cannot access it. You need to lift it to the containing context, as in my example.

Answer 2

use strict; would have found the problem.

$ perl -e'use strict; my $attribute = "..." and $attribute eq "Y";'
Global symbol "$attribute" requires explicit package name at -e line 1.
Execution of -e aborted due to compilation errors.

A my declaration only has an effect on subsequent statements, not the the statement in which the declaration is is located. (Same goes for the our and local declarations.) That means the $attribute that you create with my and to which you assign is a different variable than the $attribute you compare to Y. You want

my $attribute = $Data->{'is_new'};
if ($attribute eq 'Y') { ... }

Now, if $Data->{is_new} doesn't exist or is undefined, $attribute will be undefined, and comparing it to Y will issue a warning. You can avoid this warning as follows:

my $attribute = $Data->{'is_new'};
if (defined($attribute) && $attribute eq 'Y') { ... }

Alternatively: (5.10+)

my $attribute = $Data->{'is_new'};
if (($attribute // '') eq 'Y') { ... }