How about I explain by example:

traverse($in, 0, 0, sub {print ' ' x $_->{dept},'-',$_->{comm},"\n";})
+;

... later in the code...

traverse($in, 0, 0, sub {$sth->execute($dept, $comm)});
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By the way, the sub doesn't have to be anon:

sub _print {
   print ' ' x $_->{dept},'-',$_->{comm},"\n";
}

traverse($in, 0, 0, \&_print);
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It's a feature, it has no side effects, so there's no reason to get rid of it, and there's every reason to keep it. If you don't want to explicitely specify it, create a wrapper:

sub print_tree {
   push(@_, 0, 0, sub {print ' ' x $_->{dept},'-',$_->{comm},"\n";});
   &traverse;
}

print_tree($in);
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