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>'

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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'

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demonstrates the same oddity. However: 
perl -we'open FOO, "/dev/null"; print defined <FOO>'

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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.)

In reply to Re: matching against failed readline doesn't warn? by tilly
in thread matching against failed readline doesn't warn? by ysth

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