chdir $the_directory
    or die "Couldn't chdir to $the_directory: $!";
opendir(D, ".")
    or die "Couldn't open . ($the_directory): $!";
while ($file = readdir D)
{
    next unless $file =~ /\.las$/;
    # other logic later...
}
closedir D;

##</code><code>##

chdir $the_directory
    or die "Couldn't chdir to $the_directory: $!";
opendir(D, ".")
    or die "Couldn't open . ($the_directory): $!";
my @files = grep { /\.las$/ } readdir D;
closedir D;
foreach my $file (@files)
{
    # whatever...
}