Are you perhaps confusing a string with a list?

use strict;
use warnings;
use feature 'say';

sub uniq {  #and this to handle the duplicated emails
    return keys %{ { map { $_ => 1 } @_ } };
}

my $string = 'aaaa@jjj.net aaaa@jjj.net aaaa@jjj.net aaaa@jjj.net aaaa
+@jjj.net aaaa@jjj.net aaaa@jjj.net';
say "string: ", join ' ', uniq($string);
say "list: ", join ' ', uniq(split(/ /, $string));
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