How about:

my @emails = (split(/\s*,\s*/, $email))[0..2];
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Update: If you want to detect extras:

my @emails = split(/\s*,\s*/, $email);

if (@emails > 3) {
   ... warn ...
}

# Keep only the first three.
splice(@emails, 3);
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Update: oh, if you don't want an array:

$email = join(', ', (split(/\s*,\s*/, $email))[0..2]);
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