my %handler = (
    hello => sub() { print "hello world\n" },
);

my ($ok,$err);
my @handlers = keys %handler;
my $cfg_str = 'hello();';
{
    no strict 'refs';
    
    # We don't want to introduce another scope, as that will
    # negate the effect of the local:
    NEXT: {
        my $n = shift @handlers;
        local *{$n} = $handler{ $n };
        redo NEXT
            if @handlers;
    }
    $ok = eval $cfg_str;
    $err = $@;
}

warn $err if $err;
__END__
Undefined subroutine &main::hello called at (eval 1) line 1.