In Perl 6 a hash is list of Pair objects, so I guess your question translates to How do I turn a List into a Pair?.

In general, if you want to coerce an object of type Foo to type Bar, you call the .Bar of that object. If class Foo provides such a conversion, that is.

So if List defines a sensible conversion to Pair (which I don't know), it's as simple as

my %hash = =$fh.map: { .split(/\t/, 2).Pair };
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If it's not, you can go the way of a temporary variable, which you seem to avoid in your examples:

my %hash = =$fh.map: { my @a = .split(/\t/, 2); @a[0] => @a[1] };
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(This will store an undef as the value of the pair if there's not \t in $_).

If you don't want to use that temporary variable, you can re-use $_ in an inner lexical scope with this evil trick:

my %hash = =$fh.map: { given .split(/\t/, 2) { .[0] => .[1] } };
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This uses .[$index] to index $_ (all method calls without an explicit object work on $_).

It's not fundamentally better than the Perl 5 approach IMHO, so I'll keep thinking about a nicer solution.