open (FR, '<', '$filename') or print "Could not read $filename\n" && return;         
        my @lines = <FR>; 
        close (FR); 
         
        open (FW, '>', '$filename') or print "Could not open for write $filename\n" && return; 
        for ( @lines ) { 
            s/$ipold/$ipnew/g; 
            print FW $_; 
        } 
        close FW;