To understand it, read the man page for glob. Then understand the following:

print "$_\n" for glob "{0,1}"; # 0 1
print "$_\n" for glob "{0,1}{0,1}"; # 00 01 10 11
print "$_\n" for glob "{0,1}{0,1}{0,1}"; # 000 001 010 011 100 101 110
+ 111
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Then read up on the repetition operator in perlop, and understand:

print "AB" x 1; # AB
print "AB" x 2; # ABAB
print "AB" x 3; # ABABAB
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To use it to create keys in a hash, use something like

use Data::Dumper;
my %hash = map { $_ => 1 } glob "{0,1}" x 3;
print Dumper \%hash;
__END__
$VAR1 = {
          '011' => 1,
          '010' => 1,
          '111' => 1,
          '000' => 1,
          '101' => 1,
          '001' => 1,
          '100' => 1,
          '110' => 1
        };
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