This is not currently possible. However, the development version of perl (from 5.9.2 onwards) attempts to determine which variable was undefined; it can't always, as this example shows:

use strict;
use warnings;

my %foo;
my %bar;
$bar{'one'} = 1;
my ($x,$y);
$x = "$bar{'one'} $foo{'one'}";
$x = "$y $foo{'one'}";
$x = "xxx $foo{'one'}";
__END__
Use of uninitialized value in concatenation (.) or string at /tmp/p li
+ne 10.
Use of uninitialized value $y in concatenation (.) or string at /tmp/p
+ line 11.
Use of uninitialized value in concatenation (.) or string at /tmp/p li
+ne 11.
Use of uninitialized value $foo{"one"} in concatenation (.) or string 
+at /tmp/p line 12.
[download]

Dave.