Thanks! it is working. I have my codes as following:

use File::ReadBackwards;
use POSIX;

my $success;
my $items;
my $earliest = strftime( "%Y-%m-%d %H:%M:%S", localtime( time()- 1 * 6
+0 ) );

$bw = File::ReadBackwards->new( '/Documents/Traning/Perl/output.txt' )
+ or
                        die "/Documents/Traning/Perl/output.txt $!" ;

while ( defined( my $log_line = $bw->readline() ) ) {


    last if $log_line lt $earliest;

    if ($log_line =~ /EnableDelayedExpansion/i) {
        print $log_line;
    }else
    { print "error";}

}
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My intention is to search the string in the logfile which is still loading every min, once I found string, print out this line.