If you're a fan of lexical scoping, it could be used to improve the previous example by removing the need to undef $f in order to close the handle. With lexical scoping, when $f falls out of scope, the file is closed implicitly. See the following example:

use strict;
use warnings;
use IO::File;
{
    my $f = new IO::File "filename.txt", "r";
    while (<$f>) {
        # do your stuff
    }
}
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As the outter pair of curly brackets closes, so does the block in which $f was defined, and thus $f falls from scope, and thus is undefined, and thus the file to which it refers is implicitly closed.

Dave

"If I had my life to do over again, I'd be a plumber." -- Albert Einstein