#!perl
use strict;
use warnings;

my $foo = undef;

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

# Warning
$line = 'substitute for $foo in here';
$line =~ s/(\$\w+)/defined $1 ? $1 : ''/eeg;
print $line, "\n";