my $maxHeader = 50;
OUTER:
while ($file = <*.las>) {
    open FILE, $file or die "Couldn't open $file : $!\n";
    my $header = '';
    while (<FILE>){
        $header .= $_;
        last if $_ eq "HEADER END\n";
        if ($. > $maxHeader){
          print "Invalid file format : $file\n";
          close FILE;
          next OUTER;
        }
    }
    close FILE;
    # process $header here
}