Why doesn't my Perl grep return the first match?

Question

The following snippet searches for the index of the first occurrence of a value in an array. However, when the parentheses around $index are removed, it does not function correctly. What am I doing wrong?

my ($index) = grep { $array[$_] eq $search_for } 0..$#array;

Answer 1

The parentheses change the context in which the grep is evaluated from scalar context to list context. In scalar context grep returns the number of times the expression was true. In list context it returns the elements for which the expression was true.

The following highlights the difference context makes:

my   $x   = grep {/foo/} @array;  # the number of things that match /foo/
my  ($x)  = grep {/foo/} @array;  # the first thing that matches /foo/
my  @foo  = grep {/foo/} @array;  # all the things that match /foo/
my (@foo) = grep {/foo/} @array;  # all the things that match /foo/

Answer 2

The parentheses provide a list context for grep. grep will then actually return the list of elements for which the expression was true and not just the number of times the expression was true.