As Abigail-II has explained the functionality, here's an example of how they can come in handy. Before:

my @foo_bar;

foreach my $foo (@fooz)
{
     if ($foo > 12)
     {
          push(@foo_bar, $foo - 1);
     }
}
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Instead you could use a combination of map and grep:

my @foo_bar = map { $_ - 1 } grep { $_ > 12 } @fooz;

These constructs can be chained together and that is what makes them highly adaptable. Data flows right to left, though, so watch out.