You got your answer already to change =+ to +=. However, why do not just experiment a little bit. It is really easy to write:

use strict;
use warnings;
my $i=0;
$i=+3;
print "$i\n";  # no errors, so far, prints 3, so we should be fine...?
$i=+3;         # better try again
print "$i\n";  # still prints 3, so something must be wrong!
$i=$i+3;       # check whether Perl works correctly...
print "$i\n";  # prints 6, what a relief
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Experiments are fun! (Unless you are a theoretical physicist...)