in reply to Matching against $_ behaves differently than matching against a named scalar?

You can get the same behaviour for a named variable if you declare it outside the loop:
my $line; while ($line = <>) { ...

With the declaration inside the condition, it's in fact a different variable every time, so Perl needs to create an extra scope for it, as B::Deparse shows you:

$ perl -MO=Deparse -e 'while (<>) { /(.)/ }' while (defined($_ = readline ARGV)) { /(.)/; } -e syntax OK $ perl -MO=Deparse -e 'while (my $line = <>) { $line =~ /(.)/ }' while (defined(my $line = readline ARGV)) { do { $line =~ /(.)/ }; } -e syntax OK

Also note that lines containing 0 as their first or second word are not printed.

map{substr$_->[0],$_->[1]||0,1}[\*||{},3],[[]],[ref qr-1,-,-1],[{}],[sub{}^*ARGV,3]