thank you all..

i did some testing.. and my first idea seems to be the fastest (though i haven't tried the newer posts yet). i've created dummy lists of 100,1000,...1,000,000 emails, and this seems to take half as much time as the first reply

$i = 0;
for (@emails){
    @email = split /@/, $_;
    push @{ $domains{$email[1]} }, $i;
    $prefixes[$i] = $email[0];
    $i++;
}

foreach $key (sort keys %domains) {
    foreach ( @{ $domains{$key} } ) {
        push @sortedEmails, ( $prefixes[$_] . "@" . $key );
    }
}
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