sub mail_admin
{
  local($sub,$msg) = @_;
  $curAdmins  = "/opt/local/etc/cur_admins";

  @cur_admin_lines = `cat $curAdmins`;
  chomp(@cur_admin_lines);
  foreach $cur_admin_line (@cur_admin_lines)
  {
    ($id,$name,$group,$OncallFlag,$address) = split(/:/, $cur_admin_li
+ne);
     $cur_admin_addresses .=  " $address";
  }
  my $cmd = "echo \"$msg\" | mail -s \"$sub\" $cur_admin_addresses";
  system("$cmd");
  my($rc) = $? / 256;
  if ($rc != 0)
  {
    &pager("Can't send mail_admin message $msg - rc = $rc", P);
  }
}
[download]

• "$cmd"
  needless creates a copy of $cmd. Just use
  $cmd

• local?!
  local($sub,$msg) = @_;
  should be
  my($sub,$msg) = @_;

• What's with launching another process to read a file?
  @cur_admin_lines = `cat $curAdmins`;
  should be

  my @cur_admin_lines = do {
     open(my $fh, '<', $curAdmins)
        or die("Unable to read list of admins ($curAdmins): $!\n");
     <$fh>
  };
  [download]

• echo? print!

  my $cmd = "echo \"$msg\" | mail ...";
  system("$cmd");
  [download]

  should be

  my $cmd = "mail ...";
  open(my $fh, '|-', $cmd)
     or ...;
  print $fh $msg;
  close($fh);
  [download]

• And it's already been pointed out that $msg and $sub aren't properly converted from string to shell argument literals. Using MIME::Lite would avoid that problem.

How is that working at all?
Since you're building a scalar ($cmd) that is getting interpolated before getting called by system, the message and subject are not literals (they're interpolated as well)
What happens when $msg is this:


$msg = '"|touch file_i_shouldnt_be_allowed_to_create|"';
[download]

I imagine it's at least part of the source of your troubles (and highly dangerous from a security standpoint).