I'm not really sure, but it seems that the colon is effectively ignored in that context. Try the following: 
use v6; 

my $item = :[:[:[$hkey]]];
say $item.perl;           # outputs [[["somekey"]]]

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In reply to Re: Perl 6, defining a hash with interpolated string by molecules
in thread Perl 6, defining a hash with interpolated string by Mr_Micawber

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