I agree with hawtin about making a hash with keys based on the source and destination (reversed for replies). But what you need to gather the messages together is a hash of arrays. That will also preserve the sequence within each exchange, since I assume the log is in time order. (It will not preserve the overall sequence, but that could be another sort after you have them grouped). Here is my implementation:

use strict;
my %calls;
while (<>) {
  my $line = $_;
  my ($num, $source, $dest, $type) = split (/\s+/,$line,5);
  my $key;
  if ($type eq "GetReply") {
     $key = $dest . ":" . $source;
  } else {
     $key = $source . ":" . $dest;
  }
  push @{$calls{$key}}, $line;
}

foreach my $k (sort keys %calls) {
   print join('', @{$calls{$k}}),"\n\n";
}
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