Did you run the OP's code? Did you test your assumption? Obviously not because:

use strict;
use warnings;

my $filename = 'data.txt';

open DATA, '>', $filename or die "Can't create $filename: $!\n";

print DATA <<STUFF;
Stuff written to disk using file handle DATA
STUFF

close DATA;

open my $inFile, '<', 'data.txt' or die "Can't open $filename: $!\n";
print <$inFile>;
close $inFile;
[download]

Prints:

Stuff written to disk using file handle DATA
[download]

which would fail with the OP's error message if you were right. In fact I can't see how you get your implied error of "Can't use DATA in a file open statement" from 'Can't modify constant item in scalar assignment at date.pl line 1, near "10,"'.