my $last4 = '';
my $buffer = '';
my $find = qr /1234/;
my $chunk = 7;  # 2**20 or some such is more logical!
while ( read ( DATA, $buffer, $chunk ) ) {
    print "got last-$last4\tbuffer-$buffer\n";
    print "simple match\n" while $buffer =~ s/$find//;
    $buffer = $last4 . $buffer;
    $last4 = substr $buffer, -4, 4, '';
    print "buffer match\n" if $buffer =~ m/$find/;
}
__DATA__
1234567890123456789012345678901234567890123456789012345678901234567890

__END__
got last-	buffer-1234567
simple match
got last-567	buffer-8901234
simple match
got last-7890	buffer-5678901
got last-8901	buffer-2345678
buffer match
got last-5678	buffer-9012345
simple match
got last-8905	buffer-6789012
got last-9012	buffer-3456789
buffer match
got last-6789	buffer-0123456
simple match
got last-9056	buffer-7890123
got last-0123	buffer-4567890
buffer match
got last-7890	buffer-

##</code><code>##

my $last4 = '';
my $buffer = '';
my $find = '1234';
my $find_re = qr /$find/;
my $length = length $find;
my $chunk = 7;  # 2**20 or some such is more logical!
my $pos = 0;
while ( my $read_length = read ( DATA, $buffer, $chunk ) ) {
    $buffer = $last4 . $buffer;
    print "got last-$last4\tbuffer-$buffer\n";
    while ($buffer =~ m/$find_re/g) {
         print "match at ", $pos - (length $last4) -$length  + pos $buffer, "\n";
    }
    $last4 = substr $buffer, -$length, $length, '';
    $last4 = '' if $last4 =~ m/$find_re/;  # stop double match bug
    $pos += $read_length;
}
__DATA__
1234567890123456789012345678901234567890123456789012345678901234567890