You'd have to use something like LWP::UserAgent, which is a part of libwww.
And then you'd need some code like this:

require LWP::UserAgent;

my $ua = LWP::UserAgent->new(timeout => 30);
my $headRequest = HTTP::Request->new(HEAD => 'http://google.com/');
my $headResult = $ua->request($headRequest);
print $headResult->header('Content-Length')
[download]