I ran this as a test:

#!/usr/bin/perl

use warnings;
use strict;

my $file_base = "output";

open( FILE, ">$file_base" ) or die "Unable to open $file_base for writ
+ing!\n$!\n";

for (1..100) {
  print FILE "0" x 1024;
  print "File Size: ".(stat($file_base))[7]."\n";
  last if (stat($file_base))[7] > 50000;
}


close FILE or die "Error closing $file_base\n$!\n";
[download]

the output was as follows (abridged):

File Size: 0
File Size: 0
File Size: 0
File Size: 0
File Size: 4096
File Size: 4096
File Size: 4096
File Size: 4096
...
File Size: 49152
File Size: 49152
File Size: 49152
File Size: 53248
[download]

So it stopped close to, but not at, 50k because while the file is open, only full blocks are written. You may want to keep that in mind and make sure the cutoff is at the highest blocksize that is less than your threshold.

This is AIX 5.1 on a JFS2 file system. I won't be able to speak for others

Update:
as an aside $|++ did nothing for this
You could also keep tabs on your output size as well, if it is all under your control.

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