what you are literally asking for ('I have a long file from which I want to remove a single linebreak before all lines starting with the string "= = = =".') is this:

#!/usr/bin/perl
use strict;

my $l;

while (<DATA>) {
        if ($_ =~ /^====/) {chomp $l;}
        print $l; 
        $l=$_;
}

print $l;

__DATA__
one
two
====three
four
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