For the example like this 
#!/usr/bin/perl -w
use strict;
my %words = map { $_ => 1 } qw($Word $name);
my $line = '$Word $ $1';
$line =~ s/(\$(\w+)?)/exists $words{$1} ? $1 : "dollar$2"/eg;
print "$line\n\n";

[download]
It gives a warning message

In reply to Re^2: Use of uninitialized value in concatenation by gem555
in thread Use of uninitialized value in concatenation by gem555

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