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


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

my $sequence_number = shift || die ...;
# technically, you want 'or' instead of '||' here
...
  or die ...;
open ERROR, "/usr/bin/errpt -g -l $sequence_number |"
  or die "can't run errpt -g -l $sequence_number: $!";

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/;
}

printf SENDMAIL "%-16s: %-20s\n" => @$_ for
  ["Sequence Number" => $sequence_number],
...
  ) : (
    [ VOLSER => $message{volser} ],
  );