my @filenames = qw/a.png dpq.jpg zx.png ott.jpg zzz.tiff  oi.png/;
    foreach (@filenames) {
        my $next = next_file($_);
        say "next is $next";
    }

##</code><code>##

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;
    }
}

##</code><code>##

word is a
next is b.png
word is dpq
next is epq.jpg
word is zx
it was a z
next is zy.png
word is ott
next is out.jpg
word is zzz
it was a z
it was a z
it was a z
number of zees was 3
next is aaaa.tiff
word is oi
snakeeyes: you hit nothing with oi
next is a.png