Is the file "/opt/btb_webservice/ver3/lease_list.txt" writable ?

Mixing shell and perl without error checking is error prone.

I'd recommend using the perl module File::Inplace to replace your "sed" call, using something like (untested):

use File::Inplace;
 
my $editor = File::Inplace::->new(
    file => "/opt/btb_webservice/ver3/lease_list.txt");
while (my ($line) = $editor->next_line) {
   $line =~/$client_mac/ or next;
   $editor->replace_line(""); # Zap line
}
$editor->commit;
[download]