my @datums = (
 { field => 'out', snmptag => '1.3.6.1.2.1.2.2.1.16.1', opt => 'U' },
 { field => 'in',  snmptag => '1.3.6.1.2.1.2.2.1.10.1', opt => 'D' },
 # ...

# read 'em in (probably don't care about order, but what the heck)
for my $datum (@datums)
{
  chomp($data{$datum->{field}} = qx/snmpget -v1 -c $string $ip $datum->{snmptag}/) if $opt{$datum->{opt}};
}

$update_string = join ':', 'N', map { /: (\d+)/; $1 } @data{ map { $_->{field} } @datums };

##</code><code>##

my @fields = map {
  qx/snmpget -v1 -c $string $ip $_->{snmptag}/ =~ /: (\d+)/; $1
} grep {
  $opt{$_->{opt}}
} @datums;

$update_string = join ':', 'N', @fields;