use diagnostics or splain

$ perl -we " print $_.undef"
Use of uninitialized value $_ in concatenation (.) or string at -e lin
+e 1.
Use of uninitialized value in concatenation (.) or string at -e line 1
+.

$ perl -Mdiagnostics -we " print $_.undef"
Use of uninitialized value $_ in concatenation (.) or string at -e lin
+e 1 (#1)
    (W uninitialized) An undefined value was used as if it were alread
+y
    defined.  It was interpreted as a "" or a 0, but maybe it was a mi
+stake.
    To suppress this warning assign a defined value to your variables.

    To help you figure out what was undefined, perl will try to tell y
+ou the
    name of the variable (if any) that was undefined. In some cases it
+ cannot
    do this, so it also tells you what operation you used the undefine
+d value
    in.  Note, however, that perl optimizes your program and the opera
+tion
    displayed in the warning may not necessarily appear literally in y
+our
    program.  For example, "that $foo" is usually optimized into "that
+ "
    . $foo, and the warning will refer to the concatenation (.) operat
+or,
    even though there is no . in your program.

Use of uninitialized value in concatenation (.) or string at -e line 1
+ (#1)
[download]

The code you posted does not have a line 54