Hashes don't interpolate in double-quoted strings. That's the first reason it doesn't work. The second reason is that hashes need an even number of elements, and you're generating an odd number of elements.

To illustrate both of those points, try the following:

my %hash = ( one => 1, two => 2 );
print "%hash\n";   # See, it doesn't interpolate.
%hash = ( localtime )[5]; # Now you get a warning about uneven number 
+of elements.
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Far more than you ever wanted to know about string interpolation is described in perlop.