Anonymous Monk,
Then it sounds like the code I provided is on the right track. It would need to be modified a bit to take into account your additional requirements. Again, this is why it is important to explain your problem without assuming things are just "understood".

#!/usr/bin/perl
use strict;
use warnings;

my $string = 'One bright day in the middle of The Night';
my (%seen, $new_string);
for my $char ( split // , $string ) {
    $char = lc $char;
    next if $seen{$char} || $char !~ /^[a-z]$/;
    $seen{$char}++;
    $new_string .= $char;
    last if length $new_string == 26;
}
print "$new_string\n";
[download]

I hope this helps - L~R