Indeed in the first case you create a closure around a lexical variable. But $_ is not lexically scoped. So the anonymous sub you're putting on the right side of => prints $_ as it founds it when it is run. That is undef, hence the warning.

It's just the same with:

my @closures = map sub { $_ }, 1..5;               # won't "work"
my @closures = map { my $v=$_; sub { $v } } 1..5;  # "works"
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In this sense it seems that starting with bleædperl 5.9.1 things will be different.