Today someone asked about what the following code is doing:

my ($foo, $bar) = @{shift(@_)}{qw/ -foo -bar /};
[download]

I thought it's a good idea to add the explanation to the above idiom to the tutorial. (Although tye probably thinks that this is a bad idiom. :-)

What the above code is doing:

shift(@_)
  # get an element from the parameter list

qw/ -foo -bar /
  # creates a list of two strings: "-foo" and "-bar"

@{ shift(@_) }{ qw/ -foo -bar / }
  # dereference the anonymous hash reference as a hash slice
  # returns a list containing the values of the given keys
  # in the anonymous hash, in the form of:
  #
  #     @values = @hash{@keys}

my ($foo, $bar) = @{ shift(@_) }{ qw/ -foo -bar / };
  # assign the two element list retrieved into $foo and $bar
[download]

This technique can be used to retrieve named parameters in a subroutine. For example:

foobar({ -foo => 'FOO', -bar => 'BAR' });

sub foobar
{
    my ($foo, $bar) = @{shift(@_)}{qw/ -foo -bar /};
    print "\$foo => $foo, \$bar => $bar\n";
}
[download]

Which I think is pretty handy.

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