Your second regex in _extractInfoFromMsg() isn't matching (regex wants "NUMBER", data says "NUM"), so $1 still has the value that the first regex set it to.

You could do something like:

if ( $_ =~ /OLD TICKET NUMBER-+\[(.*?)\]/ ) {
  $fields{OLD_TICKET_NUMBER} = $1;
}
else {
  $fields{OLD_TICKET_NUMBER} = 'NOT FOUND';
}
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