From $/ in perlvar:

Remember: the value of $/ is a string, not a regex.

The difference between your two 'assignment' statements is that the first one is actually an assignment (Assignment Operators) and the second is a logical test (Equality Operators). The result of the first is assigning $/ the result of a (probably) failed regular expression, which is "". The second is a test in void context, and thus does not change the value of $/ - it remains "\n". In short, it doesn't work that way and using warnings would have told you as much.

Given that you've already constructed a regular expression, your easiest/fastest solution would be to slurp the files and use the regular expression in a while loop:

#!/usr/bin/perl
use strict;
use warnings;

local $/;
$_ = <DATA>;
while (m/(.+?(?:$|(?=\d{4}\-\d{2}\-\d{2}\s+\d{1,2}\:\d{1,2}\:\d{1,2}))
+)/gs) {
    print "START${1}END\n";
}
__DATA__
2010-03-11 18:26:42,431 DEBUG 5 System 172.22.2.120 (null) asdfasd fas
+fdsdf asdf asdf asdf asdf asdf asdf asdf asdf asdf asdf
asdf asdf asdf asdf asd fasd fasdf asdf asdf asdf as dfasdf

2010-03-11 18:26:42,431 DEBUG 9 System 172.22.2.120 (null)asdf asdf as
+df asdf asdf asdf asdf asd fasdfaf as fasdf asdf asdf as df
asdf asdf adsf sdf asdf a fasdf asdf asdfdf af

2010-03-11 18:26:42,431 DEBUG 8 System 172.22.2.120 (null)asdf asdf as
+df asdf asdf asdf asfasdfasf asf asdf asdf asdf
asd fasdf asdf asdf asdf asdf asdfasf asdf asdf
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