Using the advice of Aristotle (and, no offense intended, ignoring the advice of Abigail-II (not generally recommended)), here is some code for you. (chalk it up to a lazy Sunday ;)) 
use strict;
use warnings;

use CGI qw(a);
use URI::Find::Schemeless;

my $text   = do {local $/;<DATA>};
my $finder = URI::Find::Schemeless->new(
   sub { return a{href=>$_[0]->abs},$_[1] }
);

$finder->find(\$text);
print $text;

__DATA__
stuff stuff
http://foo.com/bar/qux.html stuff
stuff stuff
http://bar.com/baz.cgi?foo=bar stuff
stuff stuff
www.perlmonks.org/?node_id=255439

[download]
Personally, i love using CGI.pm to mark up text with HTML. Sure it may be an elephant, but it's a tried, true, and tested elephant! ;)

CORRECTION: I realized that I had the arguments to CGI::a wrong - i swapped $_[0] and $_1 to correct the mistake.

jeffa

L-LL-L--L-LL-L--L-LL-L--
-R--R-RR-R--R-RR-R--R-RR
B--B--B--B--B--B--B--B--
H---H---H---H---H---H---
(the triplet paradiddle with high-hat)

In reply to (jeffa) Re: changing existing text to a link by jeffa
in thread changing existing text to a link by Anonymous Monk

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
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