Try this. It passes your test case :) 

#!/usr/bin/perl

# https://perlmonks.org/?node_id=1232492

use strict;
use warnings;

$_ = do { local $/; <DATA> };

my @del =map /^(?=(\d+)\n)(?=.*^bar\n)/ms, /^begin\n(.*?)^end\n/gms;
my $pattern = do { local $" = '|'; qr/^\s+(foo )?ref (@del)\n/mn };
s/$pattern//gm;
print;

__DATA__
 ref 1
 ref 2
 ref 3
 ref 4
begin
1
end
begin
2
bar
end
begin
3
end
begin
4
bar
end
 ref 5
 foo ref 6
 ref 7
begin
5
end
begin
6
bar
end
begin
7
bar
end

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In reply to Re: some efficiency, please by tybalt89
in thread some efficiency, please by Anonymous Monk

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