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";
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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";
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("f_" for "formatted")

When I don't care, I prefer using:

    {
    no warnings 'uninitialized';
    print "$fum $foo\n";
    }
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But sometimes I want to track down the real cause. I'm at v5.8.7... I wonder how long until SUSE picks up perl 5.10 with the improvement?

Use of uninitialized value $foo in concatenation (.) or string at - li
+ne 6.
testing
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use diagnostics;

Update: ikegami have reason. diagnostics only give more info... but It's useful sometimes.

No, diagnostics doesn't identify which variable is the culprit. At least not the one that came with perl5.8.8.