#! /usr/bin/perl -w

use strict;

my( %totals, %delta );
my @fields = qw/foo bar/;
@totals{ @fields } = ( 10, 20 );
@delta{ @fields }  = ( 5, 6 );

# the wish
# @totals{ @fields } += @delta{ @fields };

# the reality
$totals{$_} += $delta{$_} foreach( @fields );

use Data::Dumper;
print Dumper( \%totals );