I mean if file is empty then it should add the $address in ppp.txt's first line otherwise go to the while loop

in this case, rewording and expanding Discipulus' example:

my $file = 'C:\ppp.txt';
print "file '$file' ";
if (-e $file) {
    print 'exists ';
    if (-z $file) {
        print 'but is empty';
    } else {
        print 'and is not empty';
    }
} else {
    print q!doesn't exist!;
}
print "\n";
[download]

you would have to move your code in like this (untested):

my $file = 'C:/ppp.txt';
print "file '$file' ";
if (-e $file) {
    print 'exists ';
    if (-z $file) {
         # print 'but is empty';
         my $fh, '>>', $file, or die "Could not open file '$file' for 
+appending: $!";
                 print$fh $address . ' or whatever you want';
                 close $fh;
    } else {
         # print 'and is not empty';
                 ... (your "old" open and while loop)
    }
} else {
    # print q!doesn't exist!;
        ... (either create it or print error message)
}
print "\n";
[download]