use strict;
use warnings;

sub processInterfaceData (\%\%)
{
  my ($h, $nh) = @_;

  foreach my $key (keys %{$h}) {
      print "$key\n";
  }
}

my %myUnitIfs;
my %config;
processInterfaceData($config{units}{myUnit}{interfaces}, %myUnitIfs);

##</code><code>##

Type of arg 1 to main::processInterfaceData must be hash (not hash element)