In addition, if I simply want to read the entire file into the scalar using read(), how would I do that, since I don't know the length (in bytes) to ask for?

my $buf;
$scalar .= $buf while read(FH, $buf, 4096);
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or

my $size = (stat FH)[7];
read(FH, $scalar, $size);
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As long as all processes honor your flock, there shouldn't be a race condition with the stat + read, but it still makes me uncomfortable.