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