use strict;
use warnings;

my $exten = "\\\.txt|\\\.csv";  #  \.txt|\.csv

my @files = qw(
   one.txt two.html three.csv four.log pretxt file.txt~ File.Txt
);

foreach (@files) {
  if (/($exten)\z/i) {
    print $_, "\n";
  }
}

##</code><code>##

my @exten = qw( .txt .csv );
my $exten = join('|', map quotemeta, @exten);