#!/usr/bin/perl
{
    package foo;
    sub new {
        my $type = shift;
        return bless {}, $type;
    }

    sub baz {
        print "foo-baz\n";
    }

    sub m {
        my $self = shift;
        # do some init here
        print "replace\n";
        *m = sub { $self->baz };
        $self->m();
    }
}

{
    package bar;
    use base foo;

    sub baz {
        print "bar-baz\n";
    }
}
# main
my $x = new foo;
my $y = new bar;
$x->m;
$y->m;