You need to do one of the following to make that work:

perl -e 'print "I cannot add" unless 19.08 + 2.01 == 21.09'
perl -e 'print "I can'\''t add" unless 19.08 + 2.01 == 21.09'
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BTW as Re: Strange (rounding?) problem points out, Perl often handles floats better if you use eq instead of == for testing equality...