use strict;
use warnings;
use Data::Dumper;
my $msg = 'a';
my $mhash;
$$mhash{entries}{$msg}{text} = "This is a http://foo/bar.html test.";
$$mhash{entries}{$msg}{text} =~ s!http://(\S+)![<a href="http://$1" >foo</a>]!g;
print Dumper $mhash;
__OUTPUT__
$VAR1 = {
        'entries' => {
                     'a' => {
                            'text' => 'This is a [<a href="http://foo/bar.html" >foo</a>] test.'
                            }
                     }
        };