while the use of $/ will get just the header, it's stuck in a scalar. this lets you do line-by-line processing inside the header. also, it uses labels and a precompiled regex, which i think is pretty nifty.

oh, and do you really want to die if you can't process a single file? i changed this to warn on open troubles, and die on close troubles. your mileage may vary.

#!/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";
}
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~Particle *accelerates*