Understanding "pseudo" contexts in Perl

What do you think the following lines of code print?

use strict;
use warnings;
use 5.010;

my @first  = ( 'foo', 'bar', 'baz' );
my @second = ( 'this', 'that' );
say @first + @second;
say @first . @second;

That prints 5 on one line and 32 on the second! What the heck is going on?

If you were to spend your day reading perldoc perlglossary (and who wouldn't?), you would notice this entry for scalar context:

The situation in which an expression is expected by its surroundings (the code calling it) to return a single value rather than a list of values. See also context and list context. A scalar context sometimes imposes additional constraints on the return value—see string context and numeric context.

In other words, after scalar context is applied to a variable, Perl then decides whether it should be treated as a string or a number. This is part of the reason why Perl uses the dot (.) instead of the plus (+) for concatenation: if it used the plus sign for both concatenation and addition, how would it know whether or not to treat the scalars as numbers or strings? 3 + 3 could evaluate to 6 or 33

Let's look at our example again:

my @first  = ( 'foo', 'bar', 'baz' );
my @second = ( 'this', 'that' );

say @first + @second; 
say @first . @second; 

The first say uses a plus and the second uses the dot operator. In perldoc perlop, under the second Additive Operators, it says "binary + returns the sum of two numbers" and "binary . concatenates two strings". Since we have to evaluate something in scalar context before we treat it as a string or number, the @first array, having three elements, evaluates to 3 and the @second array, having two elements, evaluates as 2. The plus operator adds them, giving us 5 for the first say, and the dot operator concatenates them, giving us 32 for the second say.

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Curtis "Ovid" Poe offers Perl training and consulting services via All Around The World, a consultancy based in France. Though having programmed in many languages, he's specialized in Perl for over a decade and wrote the test harness that ships with Perl. He recently wrote the popular book Beginning Perl (Wrox Press) and is one of the authors of Perl Hacks (O'Reilly). Ovid sits on the Board of Directors of The Perl Foundation.