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FAQ 7.13 What is variable suicide and how can I prevent it?


This is an excerpt from the latest version perlfaq7.pod, which
comes with the standard Perl distribution. These postings aim to
reduce the number of repeated questions as well as allow the community
to review and update the answers. The latest version of the complete
perlfaq is at http://faq.perl.org .

--------------------------------------------------------------------

7.13: What is variable suicide and how can I prevent it?

    This problem was fixed in perl 5.004_05, so preventing it means
    upgrading your version of perl. ;)

    Variable suicide is when you (temporarily or permanently) lose the value
    of a variable. It is caused by scoping through my() and local()
    interacting with either closures or aliased foreach() iterator variables
    and subroutine arguments. It used to be easy to inadvertently lose a
    variable's value this way, but now it's much harder. Take this code:

        my $f = 'foo';
        sub T {
          while ($i++ < 3) { my $f = $f; $f .= "bar"; print $f, "\n" }
        }
        T;
        print "Finally $f\n";

    If you are experiencing variable suicide, that "my $f" in the subroutine
    doesn't pick up a fresh copy of the $f whose value is <foo>. The output
    shows that inside the subroutine the value of $f leaks through when it
    shouldn't, as in this output:

            foobar
            foobarbar
            foobarbarbar
            Finally foo

    The $f that has "bar" added to it three times should be a new $f "my $f"
    should create a new lexical variable each time through the loop. The
    expected output is:

            foobar
            foobar
            foobar
            Finally foo

--------------------------------------------------------------------

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