while (my $ref = $sth->fetchrow_hashref())
        {
        ...
[download]

Aha! The problem there is that the $ref lives within the lexical scope of the while loop in the bar() sub. So once the while loop exits, $ref is no longer in scope. Even if it were declared about the while loop it would fall out of scope when bar() exits. What you probably intended was something like this

sub foo {
    ...

    ## get $ref back from bar()
    my($ref, %locations) = &bar($dbh);

    ...
  
    my $location_text = $locations{$ref->{'location'}};
}

sub bar {
    ...

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

    $sth->finish;

    ## return $ref back to caller
    return $ref, %locations;
}
[download]

However now $ref only contains the hash ref from the last iteration of the while loop, or is that the desired result?
HTH

_________
broquaint