Programming Perl, 4th Edition

scalar

scalar EXPR

This pseudofunction may be used within a LIST to force EXPR to be evaluated in scalar context when evaluation in list context would produce a different result. For example:

my($nextvar) = scalar <STDIN>;

prevents <STDIN> from reading all the lines from standard input before doing the assignment, since assignment to a list (even a my list) provides list context. Without the scalar in this example, the first line from <STDIN> would still be assigned to $nextvar, but subsequent lines would be read and thrown away, since the list we’re assigning to is only able to receive a single scalar value.

Of course, a simpler, less-cluttered way would be to just leave the parentheses off, thereby changing the list context to a scalar one:

my $nextvar = <STDIN>;

Since a say function is a LIST operator, you have to say:

say "Length is ", scalar(@ARRAY);

if you want the length of @ARRAY to be printed out.

There’s no “list” function corresponding to scalar since, in practice, one never needs to force evaluation in list context. That’s because any operation that wants LIST already provides list context to its list arguments for free.

Because scalar is a unary operator, if you accidentally use a parenthesized list for the EXPR, this behaves as a scalar comma expression, evaluating all but the last element in void context and returning the final element evaluated in scalar context. This is seldom what you want. The following single statement:

print uc(scalar(&foo,$bar)),$baz;

is the (im)moral ...