You could use the s operator instead of tr. Something like this:

$fcast =~ s/=\s*$//g;
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will delete a '=' followed by any amount of whitespace (including newlines) at the end of the string. If each string (this is, each element in %$forecast ends that way, you may want to apply the substitution to each element before concatenating them into $fcast.

The '=' at the end of each line sounds like some form of MIME encoding to me. If this is the case, and later you encounter that some other things come encoded in funny ways, you may want to look at the existing MIME:: packages on CPAN.