What you're finding is that mathematical operators are not overloaded to work on sets. You need something like this:

use strict;
use warnings;

use Data::Dumper;

my %hash = (
    one => [ 1, 2, 4 ], 
    two => [ 5, 7, 9 ]
);

my @add = ( 7, 8, 15 );

$hash{one}->[$_] += $add[$_] foreach 0 .. $#add;

print Dumper \%hash;
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