Technically because a slice returns a list of values not keys.

Something like exists %haystack{@slice} might be a workaround

Also the semantic with first that you propose isn't intuitive for me.

But

• @vector = map { exists $haystack{$_} } qw(aa bb cc dd)

would be the obvious choice and probably wouldn't merit magic sugar.¹

If anything this might be an argument for hyper operator syntax, which could be combined with various simple operators like exists.

Finally your goal might be possibly achieved with aliasing², I'll test and update here if it's possible.

updates

In hindsight: This is all semantically very close to calculating a set intersection, but alas not easy in Perl.

¹) though there is a precedent with delete @hash{@slice} which returns a list

²) no, doesn't work an exists $_ doesn't compile