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

##</code><code>##

BEFORE: a a
 AFTER: A A
BEFORE: A b
 AFTER: A B
BEFORE: B A
 AFTER: B A
BEFORE: B B
 AFTER: B B