Just for the TMTOWTDI of it...

#!/usr/bin/perl

use strict; # https://perlmonks.org/?node_id=11117527
use warnings;
use List::Util qw( first );

$_ = do { local $/; <DATA> };
my @fields = split /(\(BEL\))/, $_;
my $inrow = 2 + first { $fields[$_] eq '' } 0 .. $#fields;

print splice( @fields, 0, $inrow), "\n" while @fields >= $inrow;

__DATA__
Agreement(BEL)ACAP(BEL)ACAP(BEL)Accord(BEL)(BEL)albatross(BEL)(BEL)(BE
+L)albatros(BEL)(BEL)alleged violation(BEL)(BEL)(BEL)infraction présum
+ée(BEL)(BEL)allowable(BEL)(BEL)(BEL)admissible(BEL)(BEL)anchovy(BEL)(
+BEL)(BEL)anchois(BEL)(BEL)angler fish, burbot(BEL)(BEL)(BEL)lotte(BEL
+)(BEL)
[download]