5.3 Watch Your Quotes

There are some subtleties associated with the way quotes or blocks are interpreted by Perl. Consider the differences between the following statements:

$str = '$c = 10';
#
eval  $str;     # 1 
eval  "$str";   # 2
eval  '$str';   # 3
eval  { $str }; # 4

Cases 1 and 2 have identical results, and cases 3 and 4 behave identically. Can you see why? The trick is to know what the interpreter does before handing it over to eval .

In case 1, Perl gives the contents of $str to eval , just as it would for any other function. Hence eval sees the string '$c = 10' , treats it like a little program, and executes it.

In case 2, Perl does variable interpolation on the double-quoted string before handing it over to eval . Again, eval sees the contents of $str , compiles it, and executes it, assigning 10 to $c .

In case 3, the argument to eval is a single-quoted string, which is not expanded during the variable interpolation stage. For this reason, eval sees a hardcoded string (with the characters "$", "s", "t", "r") and treats it like a little program as before. As a standalone program, it is quite useless, of course. Since eval returns the result of the last expression, it returns the value of $str (the string $c = 10 ). That is, if you say,

$s = eval '$str';

$s will contain $c = 10 .

Case 4 is identical to case 3, except that the code inside the block is checked for syntax errors at compile-time (at the same time as the rest of the script).

That's all there is to know about eval . Now, let us see how to use it for expression evaluation, exception handling, and efficiency.