How about exploiting that two numbers have the same sign when their product is positive?

sub same_sign {
    $_[0]*$_[1] > 0
};
[download]

The above version will treat 0 as a singularity which never has the same sign with anything, not even with itself. If you don't want that, you can make 0 have the same sign as a positive and a negative number:

sub same_sign {
    $_[0]*$_[1] >= 0
};
[download]

... but you can't say "0 is always positive" or "0 always is negative" with my method.

Update: After thinking about it a bit, I think the <=>-trick is the following:

sub same_sign {
   ($_[0] <=> 0) == ($_[1] <=> 0)
};
[download]

because shift <=> 0 is the sign function. It has different properties again - 0 has no sign, and compares as the same sign as 0, but it's different from any positive and any negative number.