If you want the subroutine to be sensitive to whether it's being called in a scalar context or not, use wantarray.

sub foo
{
    my $n = $_[0] ? $_[0] : keys %hash;
    return wantarray() ? (keys %hash)[0..($n-1)]
                       : $n
}
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I assume your example is contrived. If not, you might check to see whether there are at least $_[0] keys.