# you can do this
$dir = 'c:\\';
opendir DIR, $dir or die "Can't open $dir perl says $!\n";
while ( $file = readdir DIR ) {
    next unless $file =~ m/\.txt$/i;
    print "Found *.txt file: $file\n";
}

# or this
$dir = 'c:\\';
opendir DIR, $dir or die "Can't open $dir perl says $!\n";
my @files = grep { /\.txt$/i } readdir DIR;
print "$_\n" for @files;

# or this
$dir = "c:\\*.txt";
@files = glob $dir;
print "$_\n" for @files;