reverse is a
chIdx is 0
a.png -> b.png
reverse is z
chIdx is 0
z.jpg -> aa.jpg
reverse is za
chIdx is 0
chIdx is 1
az.png -> ba.png
reverse is zaz
chIdx is 0
chIdx is 1
zaz.png -> zba.png
ott.jpg -> ptt.jpg
reverse is zzz
chIdx is 0
chIdx is 1
chIdx is 2
zzz.tiff -> aaaa.tiff
oi.png -> pj.png
reverse is cba
chIdx is 0
abc.jpg -> abd.jpg
$ 

##</code><code>##

sub start_magick {
    use strict;
    use warnings;
    use 5.010;
    printf "%s -> %s\n", $_, next_file($_)
      for qw/a.png z.jpg az.png zaz.png ott.jpg zzz.tiff oi.png abc.jpg/;
}

sub next_file {
    use strict;
    use warnings;
    use 5.010;
    my ($previous) = @_;
    my ( $name, $ext ) = $previous =~ /(.*)\.(.*)/;
    if ( $name !~ tr/ilo/jmp/ ) {
        $name = reverse $name;
        say "reverse is $name";
        for my $chIdx ( 0 .. length($name) - 1 ) {
            say "chIdx is $chIdx";
            substr( $name, $chIdx, 1 ) =~
              tr/abcdefghjkmnpqrstuvwxy/bcdefghjkmnpqrstuvwxyz/
              and last;
            substr( $name, $chIdx, 1 ) = 'a';
        }
        $name = reverse $name;
        $name = 'a' x ( 1 + length $name ) if $name =~ /^a+$/;
    }
    return "$name.$ext";
}