What is the difference between exists and defined in an array context

exists tests if the array slot was ever assigned to whereas defined tests if the array slot contains a defined value

print "\$fred[1] exists " if exists $fred[1];
print "\$fred[1] defined " if defined $fred[1];
$fred[1] exists
$fred[1] defined

undef $fred[1] ;
print "\$fred[1] exists " if exists $fred[1];
print "\$fred[1] defined " if defined $fred[1];
$fred[1] exists

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