index my $tring,$art

A variable declaration like my returns a value: 

c:\@Work\Perl\monks>perl -wMstrict -le
"my $x = my $y = my $z = 42;
 print $x;
"
42

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In the case of your index expression, the  my $tring sub-expression defines a scalar variable and default-initializes it with the (very well defined) undefined value undef. This value is passed to the index built-in and "promoted" to  '' (the empty string), but because you have wisely asked Perl to do so, Perl warns you about this rather suspicious transaction (update: and also the business with my). index then searches for an occurrence of the string in  $art within the empty string, and returns its success/failure. The string in  $art is never found, so index always returns -1, which is a true if-condition, and so everything gets printed.

Update: Here's an experiment: Is it possible that  '' (the empty string) is a valid hash key? If so, if you add a new artist's name to your hash as the value of the key empty-string, will this artist's name be printed in the for-loop? In other words, can index identify the empty-string of the key within the empty-string of the promoted undef? If it can, what value will index return?

Give a man a fish:  <%-(-(-(-<

In reply to Re^4: find a substring of unknow lenght if it is in a hash by AnomalousMonk
in thread find a substring of unknow lenght if it is in a hash by perlynewby

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