foreach $srcip (keys(%srciphash))
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This iterates through the keys of the hash %srciphash where each key is assigned to $srcip

foreach $dstip (keys(%{$srciphash{$srcip}}))
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This iterates through the hash stored in %srciphash associated with the the key $srcip, the cluster of curly braces is dereferencing the stored hash, then assigning the keys of that hash to $destip.

$srciphash{$srcip}{$dstip} = 1;
[download]

This assigns 1 to the value corresponding to the key $dstip which is within a hash stored in %srciphash corresponding to the key $srcip.

Hopefully this code should clarify the above statements.

my %srciphash = (
  foo => { one => undef },
  bar => { two => undef },
  baz => { three => undef },
);

foreach my $srcip(keys %srciphash) {
  foreach my $dstip(keys %{ $srciphash{ $srcip } }) {
    print "$srciphash{$srcip}{$dstip} = 1\n";
  }
}

__output__

$srciphash{foo}{one} = 1
$srciphash{baz}{three} = 1
$srciphash{bar}{two} = 1
[download]

See. perldata and perlsyn for more info on data structures and looping syntax.
HTH

_________
broquaint