Such is the evil of the standard (read UNIX) file model. One solution is as follows:

$flen = -s $file;          # file length in bytes
$from = 42;                # insertion point

open FILE, '+<the/file.ext' or die "Couldn't open file: $!";
read FILE, $buf, ($flen - $from), $from;
seek FILE, $from, 0;
print FILE 'blahblahblah', $buf;
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