You need put double quotes around the input string otherwise $to_split is just 'A'.
my $to_split = " 'A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z'";
If you use warnings you would see messages like

Useless use of a constant ("B") in void context at scrap.pl line 4.
Useless use of a constant ("C") in void context at scrap.pl line 4.
Useless use of a constant ("D") in void context at scrap.pl line 4.
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