I think your regex could be simpler. Apart from that, given what you've said about the task and the data, I'd take a stack approach to handling the input -- something like this:

#!/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 FIVECabc98765432
+10defMTCH5 and
then nothing
useful
after
that
forever
more
up to
the end
[download]

That will concatenate lines onto a stack, removing line terminations as it goes. As soon as there's a match, it's reported to STDOUT, and the stack is reset to start where the match ended.

If there's no match for an extended stretch of data, the initial line is shifted off the stack so long as the overall length of the remaining lines is enough to hold a match. (I put messages to STDERR to report this, just to see it work.)