use strict;

my %unique;
my @email = qw(
   John@foo.com
   john@foo.com
   CAPTELO@microshaft.con
   CAPTELO@microshaft.com
   captntenille@muskrat.com
   CaPtNtEnIlLe@muskrat.com
);

for (@email) {
   my $key = lc $_;
   $unique{$key} = $_ unless ($unique{$key} and $unique{$key} =~ /[A-Z]/);
}

print join("\n", values %unique), "\n";