What you've done is perfectly acceptable. You have used 'my' to declare the output variable, so it isn't global, it's lexically scoped. In the example above, it is only visible to code within the file you declare it in.

If you need to limit the scope further, you can put braces around the code which needs to see it.

package MyModule;
use File::Rsync;
my $rsync;
{
    my %rsync_out;
    $rsync=File::Rsync->new( { outfun => &rsync_out,
                               errfun => &rsync_out,
                           } );
    sub rsync_out {
      my ($message, $type)=shift;
      $rsync_out{$type}.="$message\n";
    }

    sub outmsg { $rsync_out{out} }
    sub errmsg { $rsync_out{err} }
}

# ...
# code here has no clue about %rsync_out
# (but it can still see $rsync because I declared 
# it outside the braces.)
# ...

# Access the output using the outmsg and errmsg subs
my $output = outmsg;
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The code not tested. In most cases, you don't need to bother with this anyway.