I'm trying to fill a file until I hit a certain size

I'm wondering why you wouldn't just keep up a byte count of what gets written to the file:

my $bytes_written = 0;
my $bytes_wanted = 2**30; # or whatever

my $data = "Something...\n"; # set output content here...

while (1) {
    # $data = ...;   # ... or here

    print $fh $data;
    $bytes_written += length( $data );
    last if $bytes_written >= $bytes_wanted;
}
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If you need to keep track in terms of block count, there would be a little more arithmetic involved (and correct background info about what the block size really is).

As indicated in another reply, you might need to be careful about whether length() is counting bytes or wide (utf8) characters.

UPDATED to fix stupid mistake in the "last if ..." line (needed to compare the right variable to $bytes_wanted).