I don't think the tr/// operator does what you think it does in this case. tr/// operates on characters rather than strings. Here's my approach to the problem if I understand it correctly:

use strict;
use warnings;

my $file_path='/home/sites/site18/users/rjoseph/web/images/image1.jpg'
+;
my $break_point = '/web';

my @foo = split($break_point, $file_path);
my $path = $break_point . pop(@foo);

print $path;
[download]

This prints: /web/images/image1.jpg