sub build_lazy
{
    my ($var, $callback, @params) = @_;

    
    eval <<__EVAL__;
{
    my \$$var;
    sub $var {
        \\\$var = $callback->(@params) unless defined \$$var;
        \$$var;
    };
}
__EVAL__
}

# Later ...

build_lazy( 'dbh', \&build_dbh, @dbh_params );
build_lazy( 'foo', \&build_foo );