local $/ = "HEADER END\n"; # set input record separator
while ($file = <*.las>) {
    open(FILE, $file) || die "Couldn't open $file : $!\n";
    my $header = <FILE>;
    close(FILE);
    # process $header here
}

##</code><code>##

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
}