Try 
open PROG,"program|" or die "Cannot fork";
while (<PROG>) {

 # do stuff...
}

[download]
This also depends on your called program not buffering its output.

see perlopentut

"What should it profit a man, if he should win a flame war, yet lose his cool?"

In reply to Re: not waiting for external command to complete before getting the output by Joost
in thread not waiting for external command to complete before getting the output by Anonymous Monk

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