Because print takes a list as an argument so @fred is not coerced into scalar context. In your original print @fred."\n" the concatenation coerces scalar context, hence the "10" but if you'd used a comma instead of a full stop then that would have been a list and the elements of @fred would have been printed.

$ perl -Mstrict -Mwarnings -e '
> my @fred = qw{ one two three };
> print @fred . qq{\n};
> print @fred, qq{\n};
> print qq{@fred\n};'
3
onetwothree
one two three
[download]

I hope this makes things clearer.