for the same reason strcmp() returns not just a -1,0, or 1, but a "distance" between the two strings being compared. A function or syntax has multiple uses if additional information is available to the programmer.
Example:
@array = qw(1 2 3);
$length = @array;
print $length;
By allowing a non scalar to take an implicit scalar form, we have an extremely useful and quick way to get the length of an array. Nothing that difficult to replace with a len() call, but it's language features like these that make perl ($length = @_ may be the most useful of all) so handy for quick hacks.
