#! perl
use strict;
use warnings;

my $string = '
start:
end:

test
code
start: real
1
end: real
with
start: real
repeating newlines
and more than
start: real
one

instance
end: real
of the 
start: real
desired
string


end: real


start:
end:';

my $header = 'start: real';
my $footer = 'end: real';

$string =~ /.*(^$header$(?:.*)^$footer$)/ms;

print "$1\n" if $1;