There are basically two ways. One is destructive to the original string, the other is not but means more code:

my $long_string = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ";

my $i          = 0;
my $readlength = 8;

# non destructive
while ( $i <= length($long_string) ) 
{
    my $part = substr( $long_string, $i, $readlength );
    $i += $readlength;
    
    print "$part\n";
}


# destructive
while ( $long_string =~ s/^(.{1,$readlength})// ) 
{
    my $part = $1;
    print "$part\n";
}
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