How about you read in the mail body, throw out all newlines that are preceded or followed by URI-like punctuation (like /:?&) and then throw URI::Find at it?

Something like this:

use warnings;
use strict;

use URI::Find;

my $data = join '', <DATA>;

$data =~ s/(?<=[.:\/?&])\n//g;
$data =~ s/\n(?=[.:\/?&])//g;

my $finder = URI::Find->new(
                 sub { print "Found: $_[0]\n"; });

$finder->find(\$data);

__DATA__
There's a URL http://here.com and a URL http://
there.com. What else? A really long one http://abc
.def.com?foo=bar&foo=bar&foo=bar&foo=bar&foo=bar&foo=bar&
bar=foo. And this
is a regular line break.
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