#!/usr/bin/perl
use strict;
use warnings;

my %header = ();
my $within_body = '';
my @search_strings = qw(stringa stringb stringc);
while (<DATA>) {
    chomp;
    if ($_ =~ /^\*{3} (.+?)$/) {
        $within_body = $1;
    }
    else {
        foreach my $search_strings (@search_strings) {
            if (/$search_strings/) {
                push (@{$header{$within_body}}, $search_strings);
            }
        }
    }
}

foreach my $section (sort keys %header) {
    print "Found the following strings within body *** $section\n";
    print join("\n", @{$header{$section}});
    print "\n";
}

__DATA__
*** 3993.2399
stringa
stringb
stringc

*** 3993.23384j
stringc
junk data
junk data

*** 3993.23993k
stringa
stringb
junk data
junk data

##</code><code>##

if ( @{$header{$section}} = /stringa/gm ) {
    print "matched on stringa\n";
}
if ( @{$header{$section}} = /stringb/gm ) {
    print "regex on string b\n";
}
if ( @{$header{$section}} = /stringc/gm ) {
    print "C match\n";
}

##</code><code>##

Found the following strings within body *** 3993.23384j
stringc
Found the following strings within body *** 3993.2399
stringa
stringb
stringc
Found the following strings within body *** 3993.23993k
stringa
stringb

##</code><code>##

Found the following strings within body *** 3993.23384j
C match
Found the following strings within body *** 3993.2399
matched on stringa
regex on string b
C match
Found the following strings within body *** 3993.23993k
matched on stringa
regex on string b