Consider using LWP::Simple::mirror for your specific task. There's a good example of its use in the article about LWP::Simple on the 22nd day of the 2003 Perl Advent Calendar.

Here's my own example:

#!C:/Perl/bin/perl.exe

use strict;
use warnings;
use LWP::Simple qw( mirror is_error RC_NOT_MODIFIED );

my $url  = 'http://rabbit.eng.miami.edu/dics/pocket.txt';
my $file = $url;

$file =~ s{.*/}{};

my $status = mirror($url, $file);

if (is_error($status)) {
    die "Can't get text file at URL $url\n";
}

if ($status == RC_NOT_MODIFIED) {
    warn "File $file not modified\n";
}

# Do something with freshened file named pocket.txt...

exit 0;
[download]

See 14.13 Content-Length and 14.25 If-Modified-Since for the gory details of what's going on under the hood.

Cheers!

Jim