$ cat foo.pl 
#!/usr/bin/perl -w
##################
open(OUT,">testfileA");
print OUT "This is testfileA\n";
close(OUT);

open(OUT,">testfileB");
print OUT "This is testfileB\n";
close(OUT);

open(IN,"/tmp/testfileA");

rename ("testfileB", "testfileA");

print <IN>;
close IN;