If I understand you right (??), I think you're just using the wrong syntax for the hash assignment inside the foreach. Instead of this:

foreach my $oid (sort keys(%iface)) {
  %oids = ("$iface{$oid}" => `$snmpwalk $ip $community $oid`);
}
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I think you want this:

foreach my $oid (sort keys(%iface)) {
  $oids{$iface{$oid}} = `$snmpwalk $ip $community $oid`;
}
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because I think your description said something to the effect of "keys of %iface are used as parameters in a set of back-tick operations, and values of %iface are then used as keys of of %oids -- the values in %oids are the strings returned by the back-ticks." (I'm not asserting that this is any clearer than your own description, but if you get what I mean, then I must have gotten what you meant.)