Programming Perl, 4th Edition

Line Input (Angle) Operator

The most heavily used input operator is the line input operator, also known as the angle operator or the readline function (since that’s what it calls internally). Evaluating a filehandle in angle brackets (STDIN, for example) yields the next line from the associated filehandle. (The newline is included, so according to Perl’s criteria for truth, a freshly read line is always true, up until end-of-file, at which point an undefined value is returned, which is conveniently false.) Ordinarily, you would assign the input value to a variable, but there is one situation where an automatic assignment happens. If and only if the line input operator is the only thing inside the conditional of a while loop, the value is automatically assigned to the special variable $_. The assigned value is then tested to see whether it is defined. (This may seem like an odd thing to you, but you’ll use the construct frequently, so it’s worth learning.) Anyway, the following lines are equivalent:

while (defined($_ = <STDIN>)) { print $_ }    # the longest way
while ($_ = <STDIN>) { print }                # explicitly to $_
while (<STDIN>) { print }                     # the short way
for (;<STDIN>;) { print }                     # while loop in disguise
print $_ while defined($_ = <STDIN>);         # long statement modifier
print while $_ = <STDIN>;                     # explicitly to $_
print while <STDIN>;                          # short statement modifier

Remember that this special magic requires a while loop. If you use the input operator anywhere else, you must assign the result explicitly ...