Your code does not run as is, there are errors. A simple add of an if condition is all i believe you are looking for. For testing I removed the writing to the file and printed to screen. I assumed your configuration file will only contain one set of IP constraints. Also removed the $i counter, looks like it should have been a global variable.

#!/usr/bin/perl -w

  use strict;
  use warnings;
  use Net::PcapUtils;
  use Net::Pcap;
  use NetPacket::Ethernet qw(:strip);
  use NetPacket::IP;
  #use Config::Reader::Simple;

my $file = "CaptureData.txt";
open FILE, ">$file" or die "unable to open $file $!";

my %config;
open my $config, '<', 'Config.txt' or die $!;

sub process_pkt {
      my ($user, $hdr, $pkt) = @_;
      my $ip_obj = NetPacket::IP->decode(eth_strip($pkt));
      my $eth_obj = NetPacket::Ethernet->decode($pkt);
      if($ip_obj->{src_ip} eq $config{'SourceIP'}) {
          print  "SourceIP : $ip_obj->{src_ip}\n";
          print  "SourceMAC : $eth_obj->{src_mac}\n";
          print  "EthernetType : $eth_obj->{type}\n";
          print  "IPProtocol : $ip_obj->{proto}\n";
          print  "----------------------------\n";
        }
}

while(<$config>)
{
    chomp;
    my ($key, $value) = split /\s*=\s*/, $_;
    $config{$key} = $value;
    print FILE "chave: $key -- valor: $value\n";
}


my $err ='';
my $i = 1;
my $pcap = Net::Pcap::open_offline("capture.pcap", \$err) or die "Can 
+not open file...$err\n";
Net::Pcap::loop($pcap, -1, \&process_pkt, '');

Net::Pcap::close($pcap);


close FILE, ">$file" or die "unable to close $file $!";
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