The tr/// operator (perlop).

$string =~ tr/+/ /;
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Example:

perl -e '$string = "+=+=+=+"; $string =~ tr/+/ /; print "($string)\n";
+'
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...the output...

( = = = )
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If it's possible that your data has legitimate '+' characters (perhaps escaped, or quoted), then you'll have to use a more elaborate solution.

I think that perlintro is required reading for anyone new to Perl, and while the tr/// operator isn't discussed therein, the s/// is. The s/// operator could be used (less efficiently than tr///) like this: s/[+]/ /g. So after reading through perlintro, while you might not have discovered the most efficient way of doing it, you would have gotten a good start toward coming up with a reasonable way of doing it.