First off, you need to know where the user lives, ask the system with getpwnam and friends. Then try to mount the home dir. It's up to your system's /etc/fstab (and by fstab, nsswitch as you may use NIS+ or something)

use User::pwent;

my $username = 'f00li5h';
my $user = getpwnam($username);

printf "%s aka (%s) lives in %s $/",
       $user->name, $user->gecos, $user->dir;

#f00li5h aka (f00li5h and5illy) lives in /home/f00li5h

use File::Spec;
my @paths = File::Spec->splitdir( $user->dir );

# start at the most specific directory (/home/f00li5h) head 
# to the lest specific (/)
while( @paths ){
    # get the system to mount (rather than just print)
    printf "mount %s $/", File::Spec->catdir( @paths );
    last if -d $user->dir; # stop looking when the dir turns up
    pop @paths ;
}
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This has massive massive YMMV attached to it and I'm sure there's a better way than blindly mounting things until the homedir appears, particularly if the user's homedir is with a symlink of some sort.

/home/f00li5h -> /media/homes/big/f00li5h
[download]