As evidenced by the varied responses from the other monks, your problem statement is somewhat unclear.

Perhaps this is what you're looking for?

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

while (my $myvar=<DATA>) {
    chomp($myvar);
    if ($myvar=~/^([A-Z])\1*$/) {
        print "'$myvar' matches\n";
    }
    else {
        print "'$myvar' doesn't match\n";
    }
}

__DATA__
X
XXXX
XXXY
XXYX
YXXX
AAAA
BBBB
BBBC
DDD4
[download]

Output:

'X' matches
'XXXX' matches
'XXXY' doesn't match
'XXYX' doesn't match
'YXXX' doesn't match
'AAAA' matches
'BBBB' matches
'BBBC' doesn't match
'DDD4' doesn't match
[download]