use strict;

local $/;

my $str = <DATA>;

while ($str =~ /Message dump:((?:(?!Message dump:).)*)/gs)
{
	print "$1";
}

__DATA__
Message dump:
1=4 11:13:2006
Message dump:
1=445=3=56=23=67=23=123=12=34
11:13:2006
Message dump:

output:

1=4 11:13:2006

1=445=3=56=23=67=23=123=12=34
11:13:2006