#!/usr/bin/perl

# http://perlmonks.org/?node_id=1166246

use strict;
use warnings;

my $data = <<END;
unwanted_line1=blabla
unwanted_line2=blabla
my_variable=important_content_section1
important_content_section2
important_content_section3
unwanted_line3=blabla
unwanted_line4=blabla
END

my ($getVariable) = $data =~ /(my_variable=.*\n(?:[^=\n]*\n)*)/;

print $getVariable;