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Since Perl 5001, the => automatically implies that the left argument should be interpreted as a string, making
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%hash = (key => 'value');
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perfectly legal, even with warnings and the strict pragma in force
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List Operators (rightward)
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The rightward list operators govern the interpretation of the list operator s arguments The right side of a list operator has a very low precedence, with only the and, or, xor, and not having a lower precedence This interpretation causes the problems when using implied lists and the symbolic logical operators:
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This is actually interpreted as the last list argument being logically compared with the die statement Use the named logical operators, which have lower precedence, to solve the problem
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Named Logical NOT
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The logical not provides a logical negation for the item on the right of the operator Any term on the left will immediately raise an error, so the statement
$a = $b not $c;
is completely nonsensical Use || if you want to choose between two values
Named Logical AND
This and works identically to the symbolic logical AND (&&), including the short circuit execution The only difference is that it has a lower precedence
Named Logical OR and Exclusive OR
The named logical or works like the symbolic logical OR (||), including the short circuit execution Its main benefit is that it operates at very low precedence in the lowest precedence of all statements and is therefore useful in control statements Care should be taken when using or in assignment statements Because it has the lowest precedence, the assignment operator will bind tighter than the or operator, so
$a = $b or $c;
Perl: The Complete Reference
is interpreted as
($a = $b) or $c;
It s better to write it as
$a = $b || $c;
The same is true of any other statement where you want to make comparisons the or operator is really only useful when you want to check the return value of a function without affecting the value returned The xor operator returns the exclusive OR of two expressions
Bare Words
Bare words within a script are essentially a bad idea First Perl tries to identify whether the bare word is a proper value if it can be resolved to a function within the symbol table, then the function is called; otherwise it s treated as a string The script below demonstrates this quite neatly:
sub hello { return 'Hello user!'; } $message = hello; print "$message\n"; $message = goodbye; print "$message\n";
This outputs
Hello user! goodbye
If you have warnings switched on, then Perl will warn you if it sees an all-lowercase bare word that it can t otherwise identify as a term:
3:
Perl Parsing Rules
Unquoted string "goodbye" may clash with future reserved word at tpl line 10
FUNDAMENTALS
A mixed-case bare word is interpreted as a string in most instances, and it should raise a suitable error when warnings are switched on However, there is one exception where a bare word is used in a situation that requires a filehandle, the bare word is used as the filehandle name For example, the code:
print Tester;
prints the value of $_ to the filehandle Tester, assuming it s open and writable If you try
print Tester, "Hello World\n";
you ll get an error when warnings are switched on because Perl assumes that Tester is the name of a filehandle If you have the subs portion of the strict pragma invoked, then execution will terminate because of the bare word:
Bareword "goodbye" not allowed while "strict subs" in use at tpl line 10
If you have both warnings and the strict pragma in effect, then the pragma takes precedence We ll be looking at pragmas and warnings in more detail in 8
Contexts
Perl supports a number of different contexts, which are identified for each operator or term during the parsing process The exact effects vary according to the operator or term concerned Contexts affect the operation of different statements and functions, and they are worth covering, at least briefly we ll look at the details of contexts in 6
Scalar and List Context
There are two basic contexts that all programmers are aware of: scalar and list These two contexts affect the operation of the function or operator concerned by implying the accepted value, or value returned For example:
$size = @list;
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