#!/usr/bin/perl

use strict;
use warnings;

my @stack;
my $buffer;
my $regex = qr/^.*?(.{5}abc(?:.{10})def.{5})/;
my $target_length = 26; # number of characters needed for a match

while (<DATA>) {
    chomp;
    push @stack, $_;
    $buffer = join( "", @stack );
    if ( $buffer =~ s/$regex// ) {
        my $target = $1;
        while ( $target ) {
            print "Found /$target/ after reading $. lines\n";
            $target = ( $buffer =~ s/$regex// ) ? $1 : undef;
        }
        @stack = ( $buffer );
    }
    else {
        while ( length( $buffer ) >= $target_length + length( $stack[0] )) {
            shift @stack;
            $buffer = join( "", @stack );
            warn sprintf( "No match at line %d; stack is %d lines, %d chars\n",
                          $., scalar @stack, length( $buffer ));
        }
    }
}

__DATA__
sample data
with five matches...
foo bar 5CHRSabc_TEN_CHRS1def5chrs bax qax
moo gar 5
Chrsab
c_Ten_Chrs2
de
f5Chrsnax zax 5cHrSabc_TeN_
ChRs3def5
chrs etc.
and so on
and so on
and so on
ad nausem fivecabc0123456789defmtch4 and then another FIVECabc9876543210defMTCH5 and
then nothing
useful
after
that
forever
more
up to
the end