my $file = 'output.dat';

{
   open(my $fh_out, '>', $file)
      or die("Unable to create output file \"$file\": $!\n");
 
   local *STDOUT = $fh_out;
   local *STDERR = $fh_out;

   print        "This is STDOUT (1)\n";
   print STDERR "This is STDERR (1)\n";
   warn         "This is a warning (1)\n";
}

print        "This is STDOUT (2)\n";
print STDERR "This is STDERR (2)\n";
warn         "This is a warning (2)\n";