open (INFILE,"<infile.txt");
open (OUTFILE, ">outfile.txt);

while (<INFILE>) {
 # $_ will hold one line of your file, and the loop will (slowly) go through your entire file
 
 #Mangle $_ here e.g.
 s/^\+host-1-3-1-10:/$ipnum/;

 #print it into the other file
 print OUTFILE $_;
}