If you absolutely must interpolate the constant (I'm not sure why, and what's wrong with your first solution using concatenation...), you can say:

print "This is the package => ${\__PACKAGE__} <=";
# or 
print "This is the package => @{[__PACKAGE__]} <=";
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The idea is just to create a reference to whatever you want to interpolate, and then dereference it using the double-quote-interpolated sigils ${...} or @{...}.