how to check condition and then pattern match

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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
...
          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)

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open FILE,"config.txt" or die "cannot open file : $!";
my $interface; # declare the variable
...
 
}

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my output is: 

eth0 => 192.168.8.76
lo => 127.0.0.1

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