Since while evaluates the condition in scalar context and the line input operator (<>) returns the next line of input (or undef on end-of-file) in scalar context, that code processes file line by line.

On the other hand, when <> is evaluated in list context, it returns a list of all lines (they all go the memory!) from file. 

while (<$fh>) {   # Read line by line; generally preferred way (especi
+ally for large files)
    print "The line is: $_";
}

foreach (<$fh>) { # Read all lines at once
    print "The line is: $_";
}

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One of my favorite quotes

In reply to Re: How does the while works in case of Filehandle when reading a gigantic file in Perl by reisinge
in thread How does the while works in case of Filehandle when reading a gigantic file in Perl by raj4489

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