well... look at this:

bash:~$ ./1.pl http://localhost/tmp/
Undefined
3
Use of uninitialized value at ./1.pl line 17.
Use of uninitialized value at ./1.pl line 19.
Use of uninitialized value at ./1.pl line 20.
>>text/html<< >><< >>Apache/1.3.9 (Unix) Debian/GNU PHP/3.0.14<< >><< >><<

note that this url produce an index of tmp/ directory.

and after run your script (modified):

#!/usr/bin/perl -w
use strict;
use LWP::Simple;

my @list;
my $i = 0;
foreach (head $ARGV[0]) {
    $i++;
    push @list, $_;
    print "$i: Undefined\n" unless defined $_;
}

print scalar @list, "\n";
my ($content_type, $document_length, $modified_time, $expires, $server) = @list;
print ">>$content_type<< ";
print ">>$document_length<< ";
print ">>$modified_time<< ";
print ">>$expires<< ";
print ">>$server<<\n";

get this output:

bash:~$ ./1.pl http://localhost/tmp/
2: Undefined
3
Use of uninitialized value at ./1.pl line 19.
Use of uninitialized value at ./1.pl line 21.
Use of uninitialized value at ./1.pl line 22.
>>text/html<< >><< >>Apache/1.3.9 (Unix) Debian/GNU PHP/3.0.14<< >><< >><<

So... it is a bug?
Thanks.