Question

I have code that contains:

use strict;
use warnings;
use List::Util;
my $index = first { $ARGV[$_] eq "something"; } 0..$#ARGV;

but I get

Use of uninitialized value $_ in array element at a.pl line 4.

What could cause this ?

Answer 1

As your question lacks information, I can only guess at the real problem, but I managed to get a similar error with:

C:\perl>perl -MList::Util -we "$a= first { $ARGV[$_] eq 'some' } 0..$#ARGV; print $a" foo bar some thing
Use of uninitialized value $_ in array element at -e line 1.
Can't call method "first" without a package or object reference at -e line 1.

The error does not appear if I use List::Util qw/first/ explicitly, or if I use the full package name: List::Util::first. So, my guess is that the first function is not properly imported, and does not recognize the list after the code block, leaving $_ uninitialized.

The error most likely lies elsewhere in your code.