UPDATE: Nevermind... I misread the question. I thought the variable was of length 1. It would have helped had the OP shown any input at all.

Use eq instead of a regex: 

use warnings;
use strict;

while (<DATA>) {
    chomp;
    print "ERROR $_\n" unless $_ eq 'X';
}

__DATA__
X
XA
XB

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In reply to Re: Check if a variable contains only one letter? by toolic
in thread Check if a variable contains only one letter? by Anonymous Monk

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