#!/usr/bin/perl -w
use strict;
$|++;


my $marker = qr/^__END_HEAD__$/;

FILE: while( my $file = <*.las> )
{
    open(FH, $file) or warn "Warning: can't open $file, skipping...\n" and next FILE;
    LINE: while( chomp(<FH>) )
    {
        last LINE if /$marker/;
        # ...
    }
    close(FH) or die "ERROR: can't close $file, exiting...\n";
}