use strict;
use warnings;
use feature 'say';

my @files =
(
    'log4perl.Advanced1999:06:56:12.General_Information.ErrorLog',
    'log4perl.Advanced1999:06:56:12CSS.General_Information-collector-2.ErrorLog',
    'log4perl.Advanced1999:06:56:12CSS.General_Information-collector-2.INVALID',
);

for (@files)
{
    if (/ ^ [\w_-]+ \. [\w:]+ \. [\w_-]+ \. ErrorLog $ /x)
    {
        s/ ^ ([\w_-]+) \. ([\w:]+) CSS \. /$1.$2./x;
        say;
    }
}

##</code><code>##

13:43 >perl 1783_SoPW.pl
log4perl.Advanced1999:06:56:12.General_Information.ErrorLog
log4perl.Advanced1999:06:56:12.General_Information-collector-2.ErrorLog

13:43 >