This shows the type of error you are getting:

$e = sprintf( '%s', undef );;
Use of uninitialized value in sprintf at (eval 28) line 1, <STDIN> lin
+e 20.
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And this shows how to trap that error, get access to its text (as the first argument to the $SIG{__WARN__} pseudo-signal handler routine):

eval{ 
    local $SIG{__WARN__} = sub{ 
        printf STDERR "Warning detected: %s", $_[0]; 
    }; 
    $e = sprintf( '%s', undef ); 
};;
Warning detected: Use of uninitialized value in sprintf at (eval 29) l
+ine 1, <STDIN> line 21.
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Substitute your log file handle for STDERR; and decorate or modify the output as you see fit.

See perlvar (search for "__WARN__") for more information.