use warnings;
use strict;
my @chars = ( 'a', 'b' ) ;
foreach my $char1 ( @chars )
{
  foreach my $char2 ( @chars )
  {
    print "BEFORE: $chars[0] $chars[1] \n" ;
    $char2 =~ tr/a-z/A-Z/ ;  # $char1 ALSO UPPER-CASED !?
    print " AFTER: $chars[0] $chars[1] \n" ;
  }
}
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