eth0      Link encap:Ethernet  HWaddr 00:0C:29:0F:9B:82
          inet addr:192.168.8.76  Bcast:192.168.8.255  Mask:255.255.25
+5.0
          inet6 addr: fe80::20c:29ff:fe0f:9b82/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:280252 errors:0 dropped:0 overruns:0 frame:0
          TX packets:106184 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:35501675 (33.8 MiB)  TX bytes:11696548 (11.1 MiB)
          Base address:0x1080 Memory:f4820000-f4840000

lo        Link encap:Local Loopback
          inet addr:127.0.0.1  Mask:255.0.0.0
          inet6 addr: ::1/128 Scope:Host
          UP LOOPBACK RUNNING  MTU:16436  Metric:1
          RX packets:4664 errors:0 dropped:0 overruns:0 frame:0
          TX packets:4664 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0
          RX bytes:7952486 (7.5 MiB)  TX bytes:7952486 (7.5 MiB)

##</code><code>##

open FILE,"config.txt" or die "cannot open file : $!";
my $interface; # declare the variable
while(<FILE>) {
 
 $interface = $1 if /^([\S]+)/;
 print "$interface => $1\n" if /inet addr:([0-9.]+)/;
 
}


##</code><code>##

my output is: 

eth0 => 192.168.8.76
lo => 127.0.0.1