#!/usr/bin/env perl

use strict;
use warnings;

my $start_re = qr{(?x: ^ my_variable = )};
my $end_re   = qr{(?x: (?<! ^ my_variable ) =
                     | ^ \[ [^\]]* \] $
                     | ^ [*]{3} \s+ Block \s+ \d+
                )};

while (<DATA>) {
    print if /$start_re/ .. /$end_re/ && next;
}

##</code><code>##

__DATA__
*** Block 1
unwanted_line1=blabla
unwanted_line2=blabla
my_variable=important_content_section1
important_content_section2
important_content_section3
unwanted_line3=blabla
*** Block 2
unwanted_line1=blabla
unwanted_line2=blabla
my_variable=important_content_section1
important_content_section2
important_content_section3
[stepxyz#xxxx]
*** Block 3
unwanted_line1=blabla
unwanted_line2=blabla
my_variable=important_content_section1
important_content_section2
important_content_section3
unwanted_line3=

##</code><code>##

my_variable=important_content_section1
important_content_section2
important_content_section3
my_variable=important_content_section1
important_content_section2
important_content_section3
my_variable=important_content_section1
important_content_section2
important_content_section3