my @data = (
    [ 'a', 1 ],
    [ 'a', 2 ],
    [ 'b', 3 ],
    [ 'b', 4 ]
);
my %totals;
my @order;
for (@data) {
    push @order, $_->[0] unless exists $totals{$_->[0]};
    $totals{$_->[0]} += $_->[1];
}
for (@order) {
    # Do something with $totals{$_}
}