You can't. It says which line the problem is in, and in this case, it even specifies it's in the concatenation. (String interpolation is a form of concatenation.)

You can debug it real quick as follows:

#!/usr/bin/perl -w
use strict;
use Carp;
my $foo;
my $fum="testing";
"$fum" && 0;
"$foo" && 0;
print "$fum $foo\n";
[download]

If the data is allowed to be undefined, you can used defined to check if it and sanitize it.

#!/usr/bin/perl -w
use strict;
use Carp;
my $foo;
my $fum="testing";
my $f_foo = defined $foo ? $foo : '';
my $f_fum = defined $fum ? $fum : '';
print "$f_fum $f_foo\n";
[download]

("f_" for "formatted")