You mean aside from something like i.e. untested, ...

%res = ( map { ( $_ => undef ) } grep length > 4, <DATA> ), warn keys 
+%res;
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or some such??

I'm sure something like

warn keys %{ ( map { ( $_ => undef ) } grep length > 4, <DATA> ) };
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should work, but I don't seem to have ever been able to manage it ... ah well, just goes to show how little I know and thus shouldn't come as too great a surprise :D

update

Just noticed the length is (was) wrong in my original reply ?