Already reneeb++ and ptum++ has given solutions to solve your problem.

In addition to that, if you are getting the list of files from directory, you can use '$' anchor at the end in the matching regex ($exten) to avoid files matching '.xml.bak', '.doc.bak' etc. I think this ll help you.

use strict;
use warnings;
my $exten = '(\.txt|\.csv)$';
my @files = ('one.txt.bak', 'two.html', 'three.csv', 'four.log', 'five
+.txt');
foreach (@files) {
  if (/$exten/) {
    print $_, "\n";
  }
}

prints:
three.csv
five.txt
[download]

This won't match 'one.txt.bak'.

updated: added code