Depending on the nature of your data, there are two ways, easy and hard. I interpret your question that way, that you want to have a proxy that sits in the middle between your program and the outside network.

The easy way is if you only want to intercept HTTP data. Then you can use HTTP::Proxy.

The hard way is if you want to intercept other data. Then you have to use Net::PCap together with Net::PCapUtils, which allows you to sniff and alter any TCP transmission. There are some other more convenient modules, like Net::Divert and Net::ProxyMod (by Stephanie Wehner), but these module are for BSD(-like) systems only, as they use features only found there.

Another libpcap related module you can use in Net::RawIP. It has a more OO based interface, and the documentation seems to be a little more easy to follow and quicker to get started with.

#!/usr/bin/perl

  $|=1;

  use IO::Socket ;
  use IO::Select ;
  
  my $redirect_host = 'smtp.fln.terra.com.br' ;
  my $redirect_port = 25 ;
  
  my $local_port = $redirect_port ;
  my $accept_externals = 0 ;
  
  my $log_file = 'log.txt' ;
  
######################################################################
+##########
  
  my $server = IO::Socket::INET->new(
      Listen => 5,
      ( !$accept_externals ? (LocalAddr => 'localhost') : () ) ,
      LocalPort => $local_port ,
      Proto     => 'tcp' ,
      Blocking => 1 ,
  ) ;
  
  my $client = $server->accept;
  
  my $remote = new IO::Socket::INET(
     PeerAddr => $redirect_host ,
     PeerPort => $redirect_port ,
     Proto    => 'tcp',
     Timeout  => 30) ;
     
  my %socks = (
  $remote => 'SRV' ,
  $client => 'CLT' ,  
  ) ;
     
  my %redir = (
  $remote => $client ,
  $client => $remote ,
  ) ;
  
  open (LOG,">$log_file") ;
  my $sel = select(LOG) ; $|=1; select($sel);
  
  print "Waiting client...\n" ;

  my $can_read = IO::Select->new( $remote , $client );

  my @has_buf ;

  while( @has_buf = $can_read->can_read(1) ) {
     foreach my $has_buf_i ( @has_buf ) {
       my $redir = $redir{$has_buf_i} ;
       my $buffer ;
       
       my $sel = IO::Select->new($has_buf_i) ;
       
       while( $sel->can_read(0.1) ) {
         read($has_buf_i, $buffer , 1 , length($buffer) ) ;
         if ($buffer =~ /\n$/s ) { last ;}
       }
       
       if ($buffer ne '') {
         print $redir $buffer ;
         print "$socks{$has_buf_i}>> $buffer" ;
         print LOG "$socks{$has_buf_i}>> $buffer" ;
       }
     }
  }
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