my $interface; # declare the variable
while (<>) {
 # if there are non-space symbols on the start of the line,
 # consider it interface name
 $interface = $1 if /^([\S]+)/; # $1 means first matched () group of s
+ymbols
 # a group of numeric symbols or dots after "inet addr:"
 # is considered an IP-address
 print "$interface => $1\n" if /inet addr:([0-9.]+)/;
 # use /inet6? addr: ?([0-9a-f.]+)/ if you want to match IPv6-addresse
+s
}
[download]

Thanks for the help
[download]

I have added small code which will check the condition first, if the l
+ine contain Link encap and if it found then get the ethernet address 
+and ip address other wise skip the test.

and here is my changes.
open FILE,"config.txt" or die "cannot open file : $!";
my $interface; # declare the variable
while(<FILE>) {
 if (/Link encap/){
 print "yes";
 # if there are non-space symbols on the start of the line,
 # consider it interface name
 $interface = $1 if /^([\S]+)/; # $1 means first matched () group of s
+ymbols
 # a group of numeric symbols or dots after "inet addr:"
 # is considered an IP-address
 print "$interface => $1\n" if /inet addr:([0-9.]+)/;
 # use /inet6? addr: ?([0-9a-f.]+)/ if you want to match IPv6-addresse
+s
}
}
[download]

 
But when I run the script it gives me the below output but not giving 
+the expected out.
 
"Yes"
[download]

/inet addr:([0-9.]+)/ never matches on the line where /Link encap/ does, so the address is not matched, and print is not reached. You can use your check only on the interface name matching, not on the whole loop.
Sorry if my advice was wrong.