You say you want to remove a single linebreak before all lines starting with the string "= = = =", but your snippet would remove two linebreaks ("\n\n" is replaced with nothing). Just curious about that.

Anyway, I think others have already given good ideas. Here's another one, that doesn't require holding the entire file in memory at once (unless of course the file does not actually contain any instance of "\n===="):

#!/usr/bin/perl

use strict;
use warnings;

$/ = "\n====";
while (<>) {
    s/\n====$/====/;
    print;
}
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Setting the INPUT_RECORD_SEPARATOR ($/, see perlvar) like that makes things very simple. If the file happens to have CRLF line termination, you may need to set $/ to "\r\n====" (and include "\r" in the s/// as well).

(updated upon realizing that a CRLF file would just need a modified s///; the original $/ setting above would still work fine -- oops! I just noticed that ikegami already posted this idea, as I should have known he would!)