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    <p>For most purposes, you probably won't notice a difference. However, <code>foreach</code> reads each line into a <strong>list</strong> (<a href="http://perldoc.perl.org/perlfaq4.html#What-is-the-difference-between-a-list-and-an-array%3f" rel="noreferrer">not an array</a>) before going through it line by line, whereas <code>while</code> reads one line at a time. As <code>foreach</code> will use more memory and require processing time upfront, it is generally recommended to use <code>while</code> to iterate through lines of a file.</p> <p>EDIT (via Schwern): The <code>foreach</code> loop is equivalent to this:</p> <pre><code>my @lines = &lt;$fh&gt;; for my $line (@lines) { ... } </code></pre> <p>It's unfortunate that Perl doesn't optimize this special case as it does with the range operator (<code>1..10</code>).</p> <p>For example, if I read /usr/share/dict/words with a <code>for</code> loop and a <code>while</code> loop and have them sleep when they're done I can use <code>ps</code> to see how much memory the process is consuming. As a control I've included a program that opens the file but does nothing with it.</p> <pre><code>USER PID %CPU %MEM VSZ RSS TT STAT STARTED TIME COMMAND schwern 73019 0.0 1.6 625552 33688 s000 S 2:47PM 0:00.24 perl -wle open my $fh, shift; for(&lt;$fh&gt;) { 1 } print "Done"; sleep 999 /usr/share/dict/words schwern 73018 0.0 0.1 601096 1236 s000 S 2:46PM 0:00.09 perl -wle open my $fh, shift; while(&lt;$fh&gt;) { 1 } print "Done"; sleep 999 /usr/share/dict/words schwern 73081 0.0 0.1 601096 1168 s000 S 2:55PM 0:00.00 perl -wle open my $fh, shift; print "Done"; sleep 999 /usr/share/dict/words </code></pre> <p>The <code>for</code> program is consuming almost 32 megs of real memory (the <code>RSS</code> column) to store the contents of my 2.4 meg /usr/share/dict/words. The <code>while</code> loop only stores one line at a time consuming just 70k for line buffering.</p>
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    1. POWhat's the difference between iterating over a file with foreach or while in Perl?
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    1. CONothing "fake" about the list. List is correct, array is wrong. There's no array there.
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    2. COThe distinction between arrays and lists is important. Conflating them will lead to errors in your understanding and eventually your code.
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    3. COIn Perl6 the difference will be all but nonexistent. because of lazy lists.
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