What is //= in Perl?

Question

I found an example of some Perl code I needed, but it had something in it that I didn't recognise.

my $i //= '08'; 

I can't find any reference to this anywhere! It appears to be the same as:

my $i = '08'; 

Am I missing something?

Answer 1

The //= operator is the assignment operator version of the // or 'logical defined-or' operator.

In the context of a my variable declaration, the variable is initially undefined so it is equivalent to assignment (and would be better written as my $i = '08';). In general, though,

$i //= '08'; 

is a shorthand for:

$i = (defined $i) ? $i : '08'; 

It is documented in the Perl operators (perldoc perlop) in two places (tersely under the assignment operators section, and in full in the section on 'logical defined-or'). It was added in Perl 5.10.0.

Answer 2

Short answer: It's the same as my $i = '08';.

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First, let's look at $i //= '08';

EXPR1 //= EXPR2; 

is the same as

EXPR1 = EXPR1 // EXPR2; 

except that EXPR1 is only evaluated once. It's a convenient way of writing

EXPR1 = EXPR2 if !defined(EXPR1); 

See perlop for documentation on Perl operators.

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Back to my $i //= '08';. That means

my $i; $i = '08' if !defined($i); 

but $i will always be undefined. It would be far better to write

my $i = '08';