for (@filenames) {
    s/CSS//;
}

# or written with a statement modifier:
# s/CSS// for @filenames;

##</code><code>##

for (@filenames) {
    s{           # substitute 
      CSS        # the string CSS 
      (          # and a capture consisting in
        \.       # a literal dot,
        [^\.]+   # any number of non-dots,
        \.       # another literal dot,
        ErrorLog # and the constant ErrorLog
      )          # (end of capture)
      $          # anchored at the end of the string
    }
    {$1}x;       # against the capture above
}

##</code><code>##

for (@filenames) {
    s/CSS(\.[^\.]+\.ErrorLog)$/$1/; # remove CSS from end of antepenultimate token
}

##</code><code>##

for (@filenames) {
    if (/CSS/) {
        my @t = split /\./;
        $t[-3] =~ s/CSS$//;
        $_ = join '.', @t;
    }
}