Yes I understand the code and This is my understanding the code.
while(<FILE>) {
=In this I am reading the complete file with file handle.

$interface = $1 if /^([\S]+)/;
=this line will check the one or more non whitspace char and if ti found put the value in $1 and assign to $interface.

print "$interface => $1\n" if /inet addr:([0-9.]+)/;
=this line will check the address and if it found put the value in $1.



But I am not able to check the condition on the first line.