#!/usr/bin/env perl
use v5.14;
use strict;
use warnings;
$/ = undef;
my $i = 1;
print <> =~ s/(?<=BEGINREF\s)\d*(?=ENDREF)/$i++/grimse;
=for test
BEGINREF ENDREF
foo
BEGINREF ENDREF buzz
bar
BEGINREF ENDREF.
baz
hello BEGINREF ENDREF world
BEGINREF
ENDREF
=cut