"$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.