Programming Perl, 4th Edition

eval

eval BLOCK
eval EXPR
eval

The eval keyword serves two distinct but related purposes in Perl. These purposes are represented by two forms of syntax, eval BLOCK and eval EXPR. The first form traps runtime exceptions (errors) that would otherwise prove fatal, similar to the “try block” construct in C++ or Java. The second form compiles and executes little bits of code on the fly at runtime, and also (conveniently) traps any exceptions just like the first form. But the second form runs much slower than the first form, since it must parse the string every time. On the other hand, it is also more general. Whichever form you use, eval is the preferred way to do all exception handling in Perl.

For either form of eval, the value returned from an eval is the value of the last expression evaluated, just as with subroutines. Similarly, you may use the return operator to return a value from the middle of the eval. The expression providing the return value is evaluated in void, scalar, or list context, depending on the context of the eval itself. See wantarray for more on how the evaluation context can be determined.

If there is a trappable error (including any produced by the die operator), eval returns undef and puts the error message (or object) in $@. If there is no error, $@ is guaranteed ...