My solution to this problem has been to grab File::VirtualPath. It's not standard, but at least you wouldn't have to write your own ...

# warning! untested code ahead
use File::VirtualPath;

my $p = File::VirtualPath->new();
$p->path("/home/myuser");           # or use cwd()
$p->chdir("../bob/");
print $p->path_string();            # should yield /home/bob
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And of course you could wrap all that up in a sub if you like. I like this particular solution because it works even if you're referring to a path that doesn't exist (maybe it doesn't exist yet, but it will, or it's a path to something other than a physical file in the file system).

HTH.