Can't do it with LWP::Simple (natch), but yes if you use $ua->get( $url, :content_cb => ... ) you can pass a callback which dies after the requisite number of bytes have been received.

#!/opt/local/bin/perl

use strict;
use LWP::UserAgent ();

my $ua = LWP::UserAgent->new();
$ua->env_proxy;

my $rcvd = 0;
my $buf = '';
my $resp = $ua->get( 'http://www.google.com/',
                     ':content_cb' => sub { $buf .= $_[0]; die "NO MOR
+\n" if ( $rcvd += length $_[0] ) > 512; });

print "content:\n", $buf, "\n\n";

print "headers:\n", $resp->as_string, "\n";
exit 0;

__END__
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If the callback gets too much your response object will have an X-Died header (if you care if it did or not).