use warnings;
use strict;

my $myvar;
my $take = sub {
	return unless defined $myvar;
	chomp($myvar);
	print "<<$myvar>>\n";
	undef $myvar;
};
while (<DATA>) {
	if (my ($k,$v) = /^(\w+)=(.*)$/s) {
		$take->();
		$myvar = $v if $k eq 'my_variable';
	}
	elsif (/^\[.+\]$/) {
		$take->();
	}
	else {
		$myvar .= $_ if defined $myvar;
	}
}
$take->();

__DATA__
unwanted_line1=blabla
unwanted_line2=blabla
my_variable=important_content_section1
important_content_section2
important_content_section3
unwanted_line3=blabla
unwanted_line4=blabla
unwanted_line5=blabla
my_variable=important_content_section4
important_content_section5
important_content_section6
[stepxyz#xxxx]
unwanted_content1
unwanted_line6=blabla
my_variable=important_content_section7
unwanted_line7=blabla
my_variable=important_content_section8
my_variable=important_content_section9
unwanted_line8=blabla

##</code><code>##

<<important_content_section1
important_content_section2
important_content_section3>>
<<important_content_section4
important_content_section5
important_content_section6>>
<<important_content_section7>>
<<important_content_section8>>
<<important_content_section9>>