#!/usr/bin/perl

use strict;
use warnings;

my %structure;

{
    local $/ = "";  # input record separator = empty string for "paragraph mode"
    while (<DATA>) {
        s/^(.*)\n//; # first line is key string
        $structure{$1} = $_;
    }
}

print "key: $_ / value:\n$structure{$_}\n----\n" for ( sort keys %structure );

__DATA__
first_key
Here's some data to
go with the first key

key_3
Third key gets
this part

key number 2
This element of %structure
has spaces in the hash key.