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In an SO answer daxim states:
@array ~~ $scalar is true when $scalar is in @array
to which draegtun replies:
From 5.10.1+ the order of ~~ is important. Thus it needs to be $scalar ~~ @array
How about a small primer on ~~ with link(s) to source(s) including the following specific questions: What is ~~? What is ~~ called? Why does the order matter in one version but not in a previous one?
Note that a good summary may not get all the details and can be hard to write. An introduction or primer would be very useful to save time for someone unfamiliar with ~~ while expanding the exposure of this Perlism.
Search strings: non-word-tilde-tilde non-word-at-sign.
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=~ is the Perl binding operator. It's generally used to apply a regular expression to a string; for instance, to test if a string matches a pattern: if ($string =~ m/pattern/) {
$@ The Perl syntax error or routine error message from the last eval, do-FILE, or require command. If set, either the compilation failed, or the die function was executed within the code of the eval.
The variable $$ is, as you've written, PID of current process: print $$ . "\n"; What you've written in your script is $$record{$surrogate_name} , which means accessing an element of a hash and is equivalent to $record->{$surrogate_name} . Other from that, $$name usually means dereferencing a reference to a scalar.
Answering specifically "why does the order matter in one version but not in a previous one": the smart match operator was badly designed in 5.10.0 in a way that made it difficult to use reliably, and made the given/when construct less useful than it could be, so the semantics were changed with 5.10.1 and all future versions will pretend that the 5.10.0 version never existed.
In the 5.10.1+ version of smart match, the left operand and the right operand to ~~ are always treated distinctly. Just as with the =~ regex match operator, the left side is the "subject" of the match, and the right side is the "pattern" to match against -- whether that be a plain scalar, a regex, an array or hash reference, a code reference, or whatever. The specifics are detailed pretty well in perlsyn.
You shouldn't worry about the 5.10.0 version at all unless you've already written code that depends on the 5.10.0 semantics (in which case, you should rewrite it to require 5.10.1, or else it will break on all future versions of perl).
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Smart Match, see perldoc perlsyn
Per a request in the comment, I'll give a little more: Smart Match is an operator for arbitrary data types that attempts to make sense of an equality test knowing nothing more than the types of arguments, many of the tests require complex operations like iteration and regex application
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