linux - Perl script log to file, output lag -

i have perlscript logfile parsing , executes bash command:

$messagepath = `ls -t -d -1 $dir | head -n 5 | xargs grep -l "$messagesearchstring"\`; 

i start perl script ./perlscript.pl > logfile.log.

now tail on logfile watch progress, output gets stuck every time @ line described above. output stop there seconds , continue. ???

to profile problem wrapped this:

print `date`; $messagepath = `ls -t -d -1 $dir | head -n 5 | xargs grep -l "$messagesearchstring"`; print `date`; 

the output shows command not consume lot of time:

so 6. okt 22:35:04 cest 2013 6. okt 22:35:04 cest 2013 

if run script without redirecting output file there no lag.

any idea why?

i haven't tried duplicate behaviour, might stdout buffering problem. try with:

$| = 1; $messagepath = `ls -t -d -1 $dir | head -n 5 | xargs grep -l "$messagesearchstring"`; 

update

i have tried duplicate behaviour observe: i've had make assumptions believe suspicion correct. here i'm piping, it's same redirecting file , tailing file:

./test.pl |  awk '{ print strftime("%y-%m-%d %h:%m:%s"), $0; }' 

without $| = 1, output buffered , aggregated:

2013-10-06 23:08:27 saluton, mondo: /home/lserni/test.sh 2013-10-06 23:08:27 2013-10-06 23:08:27 waiting 10s... 2013-10-06 23:08:27 saluton denove! 

with modification, each line printed generated:

2013-10-06 23:09:09 saluton, mondo: /home/lserni/test.sh 2013-10-06 23:09:09 2013-10-06 23:09:09 waiting 10s... 2013-10-06 23:09:19 saluton denove! 

i expect script doing something takes seconds, , not generating messagepath; , output delayed until perl has sizeable chunk of data send along, giving impression it's line that's stalling.

i forgot: timing pipe comes here.