use strict;
[download]

OK folks, here his code with my comments added:

sub foo {
    ...
########## Note that $ref does not exist(it's undef) #######

    my %locations = &bar($dbh);

## Note that $ref did not get passed back from bar ##
    ... 
  
########## $ref->{'location'} is undef, but 'use strict;'
########## here would have said: "can't use undef as hash"
    my $location_text = $locations{$ref->{'location'}};
}

sub bar {

    my $dbh = shift;

    my $sql_loc = qq|SELECT * FROM ndmw_location|; 
    my $sth = $dbh->prepare($sql_loc); 
    $sth->execute or die("...");

    my %locations;

########## Note that $ref IS my'ed here ###########

    while (my $ref = $sth->fetchrow_hashref())
    {
        my $id = $ref->{'id'};
        my $location =  $ref->{'location'};
        $locations{$id} = $location;
    }

    $sth->finish;


    return %locations;
########## Note that $ref goes away here ###########

}
[download]

If I throw all the code into one sub and not try and split it into sub-subs, it all works as expected.

Yes, it does work for him. That's beacuse $ref does not go out of scope.

Updated To make the problem/solution more clear.