Here is my attempt to explain part of what is happening - in the the code below:

print join( '|', %{{ }} = (1)x100 );
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{ } creates a reference to an empty anonymous hash

%{ { } } is the name of the dereferenced anonymous hash.

%{{ }} = (1) x 100 assigns 50 hash entries to the anonymous hash, before it's values are printed.

In fact, I can do the following:

use strict;
print join '|', %{{ 'X' => 1, 'Y' => 1 }} = (1)x100;
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as long as the what's inside the anonymous hash forms a legal hash, which is an empty hash in your case.

I am still scratching my head trying to figure out the %hash = (1)x100 part...

It seems

my %x;
my @x = (1) x 100;
print join '|', %x = @x;
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produces the same result...