use strict;
use warnings;

sub predchar
# modify argument in place.
# return true if no carry (done).
 {
 my $ord= ord($_[0]);
 my $nocarry;
 # define the ranges available
 my @ranges= ( [ord('0'),ord('9')], [ord('a'),ord('z')], [ord('A'),ord('Z')] );
 foreach my $range (@ranges) {
    my ($first,$last)= @$range;
    next unless $ord >= $first && $ord <= $last;  # my range?
    if ($ord == $first) { $ord= $last }
    else {
       --$ord;
       $nocarry= 1;
       }
    }
 $_[0]= chr($ord);
 return $nocarry;
 }

sub magic_decrement($)
 {
 my @chars= split ('', shift);
 for (my $loop= $#chars;  $loop>=0;  --$loop) {
    last if predchar ($chars[$loop]);
    }
 return join ('',@chars);
 }

sub test
 {
 my $s= shift || $_;
 my $result= magic_decrement($s);
 print qq{--"$s" gives "$result"\n};
 }

while (<DATA>) { chomp; test; }

__DATA__
123
abc
testbaA
ignore punct.