This is probably too simple-minded, but:

#! /usr/bin/perl -w
use strict;
use Data::Dumper;

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

sub sumup(\@\%\%)
{
 $_[1]->{$_} += $_[2]->{$_} foreach (@{$_[0]});
}

sumup(@fields, %totals, %delta);

print Dumper( \%totals );
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