Hi,
In this case, it gets interpretated as print FILEHANDLE LIST and + as unary operator.
Doing an assigment first, remove the unambiguity.

$a = $b +$c;
print $a;
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or, explicity defining the output handle:

print STDOUT $b +$c;
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But's is one of this perlish things, I recommend you using spaces, and all the things to make things more clear.
Regards,
:)