Here is a version with fileevent that works (with some problems):

# insert widget code here

open (LOG, "tail -f log.log|") || die "Nope : $!";
$mw->fileevent(\*LOG, 'readable', \&insert_lines);

MainLoop();

sub insert_lines {
    my $line;
    if(defined($line = <LOG>)){
        if( $line =~ /(CRITICAL|MAJOR)/){
            $text->insert('end',$line,$1);
        } else {
            $text->insert('end',$line);
        }
    }else{
        $mw->fileevent(\*LOG, 'readable','');
    }
}
[download]

The problem being (on my machine) that it sometimes doesn't update even there is data waiting on the pipe. Even the first time it only gets the first line of the tail, then nothing until another line is added to the file (then it gets the remainder of the tail and the newer line). You could work around this using repeat() to check the file yourself every so often -- here's a quicky example that might suffice for your needs:

# insert widget code here

open (LOG, "log.log") || die "Nope : $!";
load_file(5);
$mw->repeat(5000,\&tail_file); # check every 5 seconds

MainLoop();

sub load_file {
    my $n = shift || 10;  #how many lines to initially tail
    my @tail;
    while(<LOG>){
        shift @tail if $#tail > $n;
        push @tail, $_;
    }
    for(@tail){
        if(/(CRITICAL|MAJOR)/){
            $text->insert('end',$_,$1);
        } else {
            $text->insert('end',$_);
        }
    }
}
sub tail_file {
    seek(LOG,0,1);
    while(<LOG>){
        if(/(CRITICAL|MAJOR)/){
            $text->insert('end',$_,$1);
        } else {
            $text->insert('end',$_);
        }
    }
}
[download]

sub insert_text {
   local($_) = <LOG>;          # process 1 waiting line
   if (/(CRITICAL|MAJOR)/) {
      $text->insert('end', $_, $1);
   } else {
      $text->insert('end', $_);
   }
}
[download]

You'll have to do something else entirely (is there an 'eof' filevent? an error one?) I suspect to set yourself up to clear the readable event for that file handle, unless insert_text will be called when eof(LOG) is true.

foreach ($textline = <LOG>) {
[download]

doesn't do what you want. Perhaps:

foreach my $textline ( <LOG> ) {
# or, much better:
while(  defined( $textline= <LOG> )  ) {
[download]

$mw->fileevent( *\LOG, 'readable' => \&insert_txt )
[download]