my %REDIRECTS = (
    'www.theirsite.com'  => [qw( our.server1 theirsite/ )], 
    'www.dummy.com'      => [qw( our.server2 dummy/)], 
    # And so on
);

##</code><code>##

use URI;
my $uri = URI->new($_);
my $host = lc $uri->host; # Need to make sure the case is correct

##</code><code>##

$_ = join '/', @{ $REDIRECT{$host} };