#!/usr/bin/perl

use strict;
use warnings;
my $foo = undef;

# No warning
my $line = 'substitute for $foo in $bar here';
$line =~ s/(\$\w+)/"defined $1 ? $1 : ''"/eeg;
print $line, "\n";

##</code><code>##

hairbear% ./test2.pl
Use of uninitialized value in substitution iterator at ./test2.pl line 11.
substitute for  in  here