matching against failed readline doesn't warn?

ysth has asked for the wisdom of the Perl Monks concerning the following question:

This gives a "Use of uninitialized value in pattern match" warning:

perl -e'open FOO, "/dev/null"; use warnings; undef=~/bar/'
[download]

as does this:

perl -e'open FOO, "/dev/null"; $x=scalar <FOO>; use warnings; $x=~/bar
+/'
[download]

but this is strangely silent:

perl -e'open FOO, "/dev/null"; use warnings; <FOO>=~/bar/'
[download]

Can anyone offer an explanation?

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Re: matching against failed readline doesn't warn? by tilly (Archbishop) on Apr 08, 2004 at 06:33 UTC
I think that you can rule out the regular expression engine from being a key part of this oddity. The following pair shows the same behaviour: `perl -we'open FOO, "/dev/null";$x=<FOO>; print scalar $x' perl -we'open FOO, "/dev/null"; print <FOO>'` [download] And it isn't just print either: `perl -we'open FOO, "/dev/null"; $x=<FOO>; print $x+2' perl -we'open FOO, "/dev/null"; $x=2+<FOO>; print $x'` [download] demonstrates the same oddity. However: `perl -we'open FOO, "/dev/null"; print defined <FOO>'` [download] notices that the value is undef. UPDATE: Fixed the scalar issue that both Abigail-II and ysth pointed out to me. (It didn't make a difference in the warning behaviour.)	[reply] [d/l] [select]
Re: matching against failed readline doesn't warn? by Abigail-II (Bishop) on Apr 08, 2004 at 09:14 UTC
`perl -we'open FOO, "/dev/null"; print <FOO>'` [download] This one has `<FOO>` in list context, so an empty list is returned. No warnings when printing an empty list isn't to be taken as a surprise. Abigail	[reply] [d/l] [select]
Re: matching against failed readline doesn't warn? by sgifford (Prior) on Apr 08, 2004 at 16:43 UTC
The strangeness seems to be in the diamond operator. If I use a sub instead, I get a warning: `#!/usr/bin/perl -w open FOO, "< /dev/null" or die "No foo: $!\n"; #scalar(<FOO>) =~ /bar/ (rf() =~ /bar/) and print "bar\n"; sub rf { return scalar(<FOO>); }` [download]	[reply] [d/l]