AFAIK when you have a form with multiple fields with the same name, you will get an array, in your case with 4 entrys. try to get your entrys with

my $query = new CGI;
my @formfields = $query->param('17cardnumber');
my $entryOne = $formfields[0];
my $entryTwo = $formfields[1];
my $entryThree = $formfields[2];
my $entryFour = $formfields[3];
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