You can place the constant within parens, causing it to be checked at compiletime.

use strict;
use warnings;

use Data::Dumper;

use constant THIS => 100;

print "Testing: \n";

my %hash = ( (THIS) => 200 );

print Dumper \%hash;
[download]

Try that code with the "use constant" line commented out, and you'll see it generates a compiletime error. The error you'll get is something along the lines of:

Bareword "THIS" not allowed while "strict subs" in use at mytest.pl line 12. Execution of mytest.pl aborted due to compilation errors.