Counting variable initialization

uksza has asked for the wisdom of the Perl Monks concerning the following question:

Hello wise monks,

use warnings;
use strict;

my @l = qw/10 20 30/;
#my $t += $_ foreach @l;
my $t;
$t += $_ foreach @l;
print $t;
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I know - commented code doesn't works, but can You tell my why?

stay in peace,
Lukasz