use warnings;
use strict;
use autodie;

use Data::Dump qw(dd);

use constant TRIGGER => qr{ \b bar \b }xms;  # arms a reference number

MAIN: {  # all lexical variables within this scope are isolated

die "usage: $0 <input filename>" unless @ARGV == 1;

# slurp entire file to memory.

my $filename = $ARGV[0];

my $allfile = do {
    local $/;
    open my $fh_in, '<', $filename;
    <$fh_in>;
    };

# capture list of ref object numbers to delete.

my @object_numbers = do {

    # all these substrings appear alone on a line.
    my $rx_blk_start = qr{ ^ begin $ }xms;  # block start
    my $rx_ref_n     = qr{ ^ \d+   $ }xms;  # ref number
    my $rx_blk_end   = qr{ ^ end   $ }xms;  # block end

    # this matches any stuff before end of block.
    my $rx_not_blk_end = qr{ (?! $rx_blk_end) . }xms;

    # return list of captures.
    $allfile =~ m{
        $rx_blk_start \n
        ($rx_ref_n)   \n  # capture valid ref number for deletion
        $rx_not_blk_end*  # any stuff before block end
        ${ \TRIGGER }     # must appear at least once in block
        $rx_not_blk_end*  # any stuff before block end
        $rx_blk_end
        }xmsg;

    };
# dd 'object_numbers', \@object_numbers;  # FOR DEBUG

# build regex of ref object numbers to delete.

my ($rx_del_ref_n) =
    map qr{ \b (?: $_) \b }xms,
    join '|',
    reverse sort
    @object_numbers
    ;
# print 'delete ref n regex', $rx_del_ref_n, "\n";  # FOR DEBUG

# delete all ref n objects from text.

$allfile =~ s{ ^ (?: [ ]* foo)? [ ]+ ref [ ] $rx_del_ref_n \n }
             {}xmsgo;
# print "edited allfile [[$allfile]] \n";  # FOR DEBUG

# save processed file to new file.

my $out_filename = "$filename.removed";

open my $fh_out, '>', $out_filename;
print $fh_out $allfile;
close $fh_out;

exit;

}  # end MAIN block

die "unexpected exit from MAIN";