Actually the documentation says it returns ($content_type, $document_length, $modified_time, $expires, $server) so I'd write:

my ($content_type, $document_length, $modified_time, $expires, $server
+) = head($url);
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If you want that as a hash instead (Perl for "associative array") you could:

my %head;

@head{qw(content_type document_length modified_time expires server)} =
+ head($url);
die "Unable to get page '$url'\n" if !defined $head{content};
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which uses a hash slice to initialize the key/value pairs. Note the % sigil which tells the viewer that head returns a hash. Access the fields like:

my $modified_time = $head{modified_time};
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Now see that we use a $ sigil to show that head{modified_time} returns a scalar value. The sigil ($, @, % or &) tells you what type of thing the expression returns. See perldata for extensive documentation on Perl's data structures and variable naming etc.