I stopped improving nixtFile after seeing the results (the rules I don't get)

#!/usr/bin/perl --
## 
## 
## 
## perltidy -olq  -csc -csci=3 -cscl="sub : BEGIN END " -otr -opr -ce 
+-nibc -i=4 -pt=0   "-nsak=*"
## perltidy -olq  -csc -csci=10 -cscl="sub : BEGIN END if " -otr -opr 
+-ce -nibc -i=4 -pt=0   "-nsak=*"
## perltidy -olq  -csc -csci=10 -cscl="sub : BEGIN END if while " -otr
+ -opr -ce -nibc -i=4 -pt=0   "-nsak=*"
#!/usr/bin/perl --
use strict;
use warnings;
use Data::Dump qw/ dd /;

exit newTest();

sub newTest {
    my @list;
    my @filenames = qw/a.png dpq.jpg zx.png ott.jpg zzz.tiff  oi.png/;
    for my $curr ( @filenames ) {

#~         my $next = next_file( $curr );
#~         my $nixt = nixtFile( $curr );
        my $next = next_file( "$curr" );
        my $nixt = nixtFile( "$curr" );
        my $eq   = int( $next eq $nixt );
        dd( [ $curr, $eq, $next, $nixt ] );
        push @list, [ $curr, $eq, $next, $nixt ];
    }
    dd( \@list );
} ## end sub newTest

sub nixtFile {
    my( $word ) = shift;
    my( $file, $ext ) = $word =~ m{(.+?)(\.[^.]+)?$};

#~     dd( $file, $ext );
    my @alphabet = qw/a b c d e f g h j k m n p q r s t u v w x y z/;
    my %next = map {
        my $ix = ( $_ + 1 ) % @alphabet;
        @alphabet[ $_, $ix ];
    } 0 .. $#alphabet;

#~     die dd( \%next );

    my $newfile = '';
    for my $char ( split //, $file ) {
#~         my $newchar = $next{$char} || 'FUDGE';
        my $newchar = $next{$char} || $char;
        $newfile .= $newchar;
    }
    return $newfile . $ext;
} ## end sub nixtFile

sub next_file {
    use strict;
    use warnings;
    use 5.010;
    my ($previous) = shift;
    my @alphabet = qw/a b c d e f g h j k m n p q r s t u v w x y z/;
    $previous =~ /(.*)\.(.*)/;
    my $word = $1;
    my $ext  = $2;
    say "word is $word";
    my @letters = split( //, $word );
    my $z = 0;
    # main control outer loop...all indexes begin with 0
    foreach my $dummy ( 0 .. $#letters ) {
        foreach my $index ( 0 .. $#alphabet ) {
            if ( $alphabet[$index] eq $letters[$dummy] ) {
                my $highest = $#alphabet;
                if ( $index < $highest ) {
                    $letters[$dummy] = $alphabet[ $index + 1 ];
                    my $string = join( '', @letters, ".$ext" );
                    return $string;
                }
                else {
                    say "it was a $alphabet[-1]";
                    $z = $z + 1;
                }
            }
        }
    }
    if ( $z eq ( $#letters + 1 ) ) {
        
        say "number of zees was $z";
        my $newindex = $#letters + 1;
        my @newword;
        foreach ( 0 .. $newindex ) {
            $newword[$_] = $alphabet[0];
        }
        my $newstring = join( '', @newword, ".$ext" );
        return $newstring;
    }
    else {
        say "snakeeyes: you hit nothing with $word";
        my $string3 = join( "", $alphabet[0], ".$ext" );
        return $string3;
    }
}

__END__
word is a
["a.png", 1, "b.png", "b.png"]
word is dpq
["dpq.jpg", 0, "epq.jpg", "eqr.jpg"]
word is zx
it was a z
["zx.png", 0, "zy.png", "ay.png"]
word is ott
["ott.jpg", 0, "out.jpg", "ouu.jpg"]
word is zzz
it was a z
it was a z
it was a z
number of zees was 3
["zzz.tiff", 0, "aaaa.tiff", "aaa.tiff"]
word is oi
snakeeyes: you hit nothing with oi
["oi.png", 0, "a.png", "oi.png"]
[
  ["a.png", 1, "b.png", "b.png"],
  ["dpq.jpg", 0, "epq.jpg", "eqr.jpg"],
  ["zx.png", 0, "zy.png", "ay.png"],
  ["ott.jpg", 0, "out.jpg", "ouu.jpg"],
  ["zzz.tiff", 0, "aaaa.tiff", "aaa.tiff"],
  ["oi.png", 0, "a.png", "oi.png"],
]
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