You could get the external program (or a wrapper for it) to print the file's final size to STDOUT. Then replace system() with backticks, eg 
my $size = `rsh remotehost external_program`;
chomp $size;
sleep 1 while (! -f $file or -s $file < $size);

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In reply to Re: Waiting for delayed output after system() by dave_the_m
in thread Waiting for delayed output after system() by to.b

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