jasonk@critter% cat foo.pl

#!/usr/bin/perl -w
##################
open(OUT,">/tmp/testfileA");
print OUT "This is testfileA\n";
close(OUT);

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

open(IN,"/tmp/testfileA");
system("mv /tmp/testfileB /tmp/testfileA");

print <IN>;                                                                     

jasonk@critter% ./foo.pl

This is testfileA