use strict;
use warnings;

my $file1 = 'file1.dat';
my $file2 = 'file2.dat';
for my $file ( $file1, $file2 ) {
  open my $fh, '>', $file or die;
  print {$fh} "Test data for file '$file'\n";
  close $fh or warn;
}
rename $file1, $file2 or die;