Question

Code:

my $tmp='x $i y';  # define a macro or whatever

for my $i (0..5){
    my $var;
    #    eval { $var=$tmp; };       # A
    #    eval { $var="x $i y"; };   # B
    $var="x $i y";                  # C
    print $var."\n";
}

B and C would print

x 0 y
x 1 y
x 2 y

A print

x $i y
x $i y
x $i y

What’s wrong in A?

Answer 1

eval BLOCK only catches exceptions, so it’s not relevant to the question. So that leaves us with the following code:

my $i;
my $tmp = 'x $i y';
$i = 3;
my $var = $tmp;

The right-hand side of the assignment is the same string in both cases, so why do you expect $tmp and $var to end up with different values? That makes no sense.

So how does one solve your problem? If you want the contents of $tmp to be used as a Perl code, you need to use eval EXPR, and the content of $tmp actually has to be Perl code.

my $i;
my $tmp = '"x $i y"';   # Quotes added to make it valid Perl.
$i = 3;
my $var = eval $tmp;    # eval EXPR instead of eval BLOCK.

But that’s oh-so-wrong. You’re using eval EXPR as template system. Use a real tempalte system, like Template-Toolkit. But if you want to keep using templates of the form x $i y, then look into String::Interpolate.