map can't handle recursion.

Of course you can have a map block inside a function, and recurse into that from inside the map block:

sub hash_filter {
    my $source = shift;
    my $filter = shift;

    return {
        map {
            ref $filter->{$_} eq 'HASH'
            ? ref $source->{$_} eq 'HASH'
              ? ($_, hash_filter( $source->{$_}, $filter->{$_} ))
              : croak "bad filter: on '$_', expected HASH\n"
            : ($_, $source->{$_})
        } grep {
            exists $source->{$_}
        } keys %$filter
    }
}
[download]

which btw is the for loop of the OP rewritten in terms of map/grep.

update: a terse version which eliminates shifting @_ and working directly on the arguments (which aren't altered by the function):

sub hash_filter {
    return {
        map {
            ref $_[1]->{$_} eq 'HASH'
            ? ref $_[0]->{$_} eq 'HASH'
              ? ($_, hash_filter( $_[0]->{$_}, $_[1]->{$_} ))
              : croak "bad filter: on '$_', expected HASH\n"
            : ($_, $_[0]->{$_})
        } grep { exists $_[0]->{$_} } keys %{$_[1]}
    }
}
[download]

Now that's arguably micro-optimized and far less readable (11 lines) than the OP's code (18 lines) imho.

More elegant? Perhaps for those who prefer nested ?: statements for simple expressions over if/else blocks...

update 2: B::Concise shows identical optrees (execpt line numbers) for the "for" and "map/grep" solutions.

Thanks for the example of how this can be done. I was scratching my head trying to figure out how to handle the recursion with map.

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