Initializing perl variables using eval

Question

I\'m guessing this should be something obvious to those knowing Perl, but I simply don\'t get it.. I also guess it has to do with problems described in Perl scoping « darkne

Answer 1

The problem is that when you do eval $string, $string is evaluated as its own subroutine which has its own lexical scope. From perldoc -f eval:

In the first form [in which the argument is a string], the return value of EXPR is parsed and 
executed as if it were a little Perl program. The value of the expression (which is itself 
determined within scalar context) is first parsed, and if there were no errors, executed in the 
lexical context of the current Perl program, so that any variable settings or subroutine and format 
definitions remain afterwards.

So, in other words, if you have:

use strict;
use warnings;
eval "my $foo=5;";
print "$foo\n";

you'll get an error:

Global symbol "$foo" requires explicit package name at -e line 3.
Global symbol "$foo" requires explicit package name at -e line 4.

However, if you initialize your variables first, you're fine.

use strict;
use warnings;
my $foo;
eval "\$foo=5;";
print "$foo\n"; #prints out 5, as expected.