Except that $string++ not only doesn't generate errors or warnings, but is in fact a highly useful DWIMism of Perl:

use warnings;
use strict;

my $letter = 'a';
print $letter++ for 1 .. 26;
print "\n";

my $serial = 'S001';
printf "%s, ", $serial++ for 1 .. 10;
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Prints:

abcdefghijklmnopqrstuvwxyz
S001, S002, S003, S004, S005, S006, S007, S008, S009, S010,
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