unless ($^O =~ /aix/) { die ... }
# might be better as


die ... unless $^O =~ /aix/;

##</code><code>##

my $sequence_number = shift || die ...;
# technically, you want 'or' instead of '||' here

open (ERROR, "/usr/bin/errpt -g -l $sequence_number |");
# you don't check to see if it opened properly...
# nor do you ensure the safety of the variable
# if I enter '; rm -rf' here, you're in trouble


my ($sequence_number) = shift =~ /(\d+)/
  or die ...;
open ERROR, "/usr/bin/errpt -g -l $sequence_number |"
  or die "can't run errpt -g -l $sequence_number: $!";

##</code><code>##

while (<ERROR>) {
  $message{host} = (split)[1], next if /^el_nodeid/;
  $message{drive} = (split)[1], next if /^el_resource/;
  $message{detail} = (split)[1], next if /^el_detail_data/;
}

##</code><code>##

printf SENDMAIL "%-16s: %-20s\n" => @$_ for
  ["Sequence Number" => $sequence_number],
  [ Host => $message{host} ],
  [ Drive => $message{drive} ],
  [ Model => $message{model} ],
  [ Microcode => $message{mml} ],
  [ "Message Type" => $message{type} ],
  [ "Message Code" => $message{code} ],
  [ Severity => $message{severity} ],

  $message{type} eq "SIM" ? (
    [ "First FSC" => $message{first_fsc} ],
    [ "Last FSC" => $message{last_fsc} ],
  ) : (
    [ VOLSER => $message{volser} ],
  );