Re^2: Modification of a read-only value

[Sprad]

Yes! This was the problem. I'm not sure how far back I'm picking up constant values, but I changed the "while (<PIPE>)" to "while ($foo = <PIPE>)" and that took care of it. Thank you!

I didn't realize that whiles treated $_ differently. I'll watch that in the future...

[Aristotle]

:)

I first saw this in a question in the monastery a long time ago. That was a far nastier case, though: someone was getting spurious action at a distance. Turns out one of his map blocks was calling a sub containing a while(<FH>), and because map aliases $_, the function modified the values in his source array. Ouch.

Ever since, while(<FH>) used without a local has been a red flag in my mind. Always localize $_ when you use this construct. (Or use a different variable, as you did, of course.)

[ikegami]

while (<PIPE>)
is equivalent to
while (defined($_ = <PIPE>))
so you might want to use
while (defined($foo = <PIPE>))
instead of
while ($foo = <PIPE>)
What you are using will stop when you don't expect it (such as when you read in a blank line).

[Aristotle]

Perl's one step ahead of you:

$ perl -MO=Deparse -e'while($foo=<>){}'
while (defined($foo = <ARGV>)) {
    ();
}
-e syntax OK

This is a relatively recent change from two or so years ago, though; I don't rememeber which version introduced it off-hand.

Even without the definedness test it wouldn't stop on a blank line either, btw. Don't forget the record separator. A blank line is not an empty string; it's "\n". What that construct would fail to process is the very last line of a file, if it contains only a 0 and is not terminated by a record separator. (It can't be blank, obviously, because a blank line with no record separator is, well, err, no line at all. :-))