The perldoc on keys clearly states

Called in list context, returns a list consisting of all the keys of the named hash…

Googling around for an explanation also yielded no results.

Put very simply, why does

        sub uniq {
            my %hash = map { $_ => 1 } @_;
            return keys %hash;
        }
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        sub uniq {
            return keys map { $_ => 1 } @_;
        }
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Edit:

I express my sincerest thanks to the monks for a warm welcome into the monastery.

Following the suggestions, I tried

        sub uniq {
            # two pairs of braces around map
            return keys %{ { map { $_ => 1 } @_ } };
        }
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However, prior to this, even before I posted the question, I experimented with

        sub uniq {
            # single pair of braces around map
            return keys %{ map { $_ => 1 } @_ };
        }
[download]

I had always believed that braces can convert a list with even number of elements into a valid HASHref, and modulo (%) can convert any valid HASHref into a hash.

The question now is this: if map just produces a list, why does it need two pairs of braces to emit a valid HASHref?