A first go at it, that will work even if the ids are not sorted (assuming we start with $var):

my %hash;
$hash{$_->{id}} += $_->{value} for @$var;
$var = [ map { +{id => $_, value => $hash{$_} } } sort keys %hash ];
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Second go, based on your initial assumption, possibly more efficient but uglier for sure:

my $lastind = -1;
for my $i (0 .. @$var - 1) {
   if ($var->[$i]->{id} eq $var->[$lastind]->{id}) {
        $var->[$lastind]->{value} += $var->[$i]->{value};
        splice @$var, $i, 1;
    } else {
        $lastind = $i;
    }
}
[download]