You cannot in general. With non-seekable handles (pipes,
STDIN, sockets, fifo, etc) you can't seek back, and usually
you can't put something back in the stream either.
But you can use a buffer. The simplest is a one-scalar
buffer. If you want to look, read in the line in a scalar,
later, if you want to process the line, process the scalar.
You can wrap this up in a module, and if you want to be
fancy, you tie it to a filehandle. Or with perl 5.8, you
should be able to write an IO layer that does that for you.
Abigail
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