What's the best/smartest way to grab lines from a file and discard them from the file after they've been used?

As already indicated by beable, here is good solution:

#!/usr/bin/perl -w 
###############################################
use strict;
use Tie::File;
my @ry=();

tie @ry,"Tie::File","file2use.txt" or die $!;

my $line = shift @ry;

untie @ry;
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An alternative way that has a better performance (as it uses less expansive file operations):

1. Instead of using one file, use a series of files. Numbering them, so they come in sequence.
2. have a size limitation on files. You always append to the last file, but when it reaches its size limitation, create a new file.
3. when you read, you always read from the first file, and remember the last line you read, next time, just continue from there. Once all lines are read, delete the first file, and go to the new first (by having some simple numbering mechanism, you will always know which file is the first.)

If you can accept a fixed number of fixed sized entries you could (with appropriate locking, of course) make a ring buffer. Store the head and tail offsets as the first record and then use something along the lines of seek( RING, $rec_num * REC_SIZE() + OFFSETSIZE(), SEEK_SET() ) and sysread( RING, $buf, REC_SIZE() ) to retrieve them.

So if you do this kind of stuff on small files, Tie::File is a fine solution. If the files get big, you'd better rethink your storage format. E.g. use multiple files with a given maximum size, or use a dbm file, or go for a SQL database or so. Or if you don't mind that old data remains in the file, simply have a companion file that remembers the seek position of where you got last time, or even have this data in a fixed size record at the start of your file (there are ways to overwrite a range of bytes with the same amount of (different) bytes.