Let Perl do the bookkeeping:

you can populate your arrays with push, no need for counting:

push @attach_list, 'Bach.jpg';
push @attach_list, 'workshop notes.pdf', 'Booking form.pdf';
push @attach_list, qw(testaaa.xls testx.xlsx wordtest.docx); # qw() on
+ly if the file names don't have spaces in them
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(you'll see the advantage when you shuffle the attachments, or remove one in the middle...)

Likewise, there is no need for loop counters, if you use a "foreach" loop:

for $to_em (@to_email_list) {
   ...
   $smtp->to($to_em);
   ...
}
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