use strict;
use warnings;
use feature 'say';

use constant LETTERS => join '', 'A'..'Z', 'a'..'z';    # template
use constant N => 51;                                   # number of "edges"

die if not N % 2        # (1) prohibit odd nodes number
    or N > 51;          # (2) "template" limitation

say '_' . substr LETTERS, 0, N;
my $last = substr LETTERS, N - 1, 1;    # "collect" last line here

for ( 1 .. N - 1 ) {
    my $s   = substr LETTERS, 0, N;
    my $sfx = substr $s, 0, $_ - 1, '';
    $s  .= $sfx;                        # rotate left       
    $sfx = substr $s, $_, 1, '_';    
    $s  .= $sfx;                        # ...& append replaced diagonal item

    say $s;
    $last .= substr $s, -1;
}

say $last . '_';

__END__