i want to take a hash of simple regex ie(0-9, a-z) and match them in square brackets/box brackets

Under these conditions, your "example 6" would look sth. like:

use strict;
use warnings;

my %Build = (
   jj => '0-9',
   kk => 'a-z',
);

 my $message = '[445] [5353453] [fff] [445]';
 my $message6 = $message;
 
 while( my ($k,$v) = each %Build) {
    my %speed_fix;
    if(my @re_match = $message6 =~ m{ (?<=\[) [$v]+ (?=\]) }xisg ) {
       foreach my $m_check (@re_match) {
          next if $speed_fix{$m_check};
          if( my $built = return_number($m_check) ) {
             $message6 =~ s{ \[ $m_check \] }{$built}xisg;
             ++$speed_fix{$m_check};
          }
       }
    }
 }
 print "$message6\n";
 
sub return_number {
 my $numbers = shift;
 return $numbers;
}
[download]

Maybe one could delete more stuff - if you could provide an exact specification of the problem.

Regards

mwa