Well, I can think of *one* reason why you might not want to do this:

U:\>perl -e "package bar; sub mankato {print __PACKAGE__}; package foo
+; *mankato
 = \&bar::mankato; mankato();"
bar
[download]

Update: I guess a slightly better example would be:

U:\>perl -we "package bar; sub new {return(bless({}))}; sub mankato{pr
+int __PACKAGE__}; package foo; push @ISA, qw|bar|; package main; my $
+o = foo->new(); $o->mankato();"
bar
[download]