First, the simple way to get what you want:

my $str = 'A2ABA45';
my @parts = $str =~ /A[^A]*/g;
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or

my @parts = split /(?=A)/, $str;
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To explain why you have undef's in your first attempt

use Data::Dumper;
my @parts = split /(A.+?)(?=A|$)/, "A2ABA45";
print Dumper(\@parts);
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You're treating your wanted text as delimiters that are captured in the split. Just look at this example.

my @parts = split /(-)/, "-1-2-";
print Dumper(\@parts);
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