#!/usr/bin/env perl

use strict;
use warnings;

my $comment_line_re = qr{ \A \s* [#] }x;

while (<DATA>) {
    print "@{[/$comment_line_re/ ? 'IS' : 'NOT']} Comment: $_";
}

__DATA__
# This file is a comment and should not be used
 # This comment has one leading space
   # This comment has three leading spaces
/dev/mapper/centos-var_log /var/log                xfs     defaults        1 2
 /dev/donstest  /expert         none    1 1 -rw,noglob # this is a fs
 #/dev/donstest22   /expert         none    1 1 -rw,noglob # this is a fS