What's the exact contents of the two variables, char by char, byte by byte? And, post the contents of /dm/file_names.txt, best as a hex (or similar) dump.
Given $file_date's contents, as you post, is
201701
201702
201703
201704 and $cur_yr_month's is
201701
201702
201703, of course "the if condition is not satisfying" as the two don't match.