gives the line number of the line with the shift command.

I'm not sure what would cause that, but the warning is indeed originating from "$item". $item contains undef, either because the first element of @data was undef before the shift, or because @data was empty before the shift.

By the way, shifting out of @data will not effect @{$hoa{$head}}. You need to shift from @{$hoa{$head}} directly to do that.

$\ = "\n";

{
   my %hoa = ( head => [ 'abc', 'def' ] );
   print shift(@{$hoa{head}});   # abc
   print @{$hoa{head}};          # def    -> ok
}

{
   my %hoa = ( head => [ 'abc', 'def' ] );
   my $data = $hoa{head};
   print shift(@$data);          # abc
   print @{$hoa{head}};          # def    -> ok
}

{
   my %hoa = ( head => [ 'abc', 'def' ] );
   my @data = @{$hoa{head}};
   print shift(@data);           # abc
   print @{$hoa{head}};          # abcdef -> BAD!
}
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#!/usr/bin/perl
use strict;
use warnings;

use Data::Dumper;

my %hoa = (
    a => ["apple", "aardvark"],
    b => ["banana", "bandana"],
);
        
foreach my $head (keys %hoa) {
    my @data = @{$hoa{$head}};
    my $item = shift(@data);
    print "$item,";
}

print $/;
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apple,banana,
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Try pushing the values onto your array:

my @data;
foreach $head (keys %hoa) {
    push @data, $hoa{$head};
    $item = shift(@data);
    print OUTFILE "$item";
    print OUTFILE ",";
}
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For your specific example, this would work just as well:

print OUTFILE join ',' values %hoa;
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foreach $head (keys %hoa) {
    my @data = ( $hoa{$head} );
    $item = shift(@data);
    print OUTFILE "$item";
    print OUTFILE ",";
}
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Update: I just realized what you were trying to do :) I'm assuming %hoa is actually a hash of arrayrefs, right? I'm guessing that something is actually going on with the construction of your HoA such that your hash entries are actually pointing at undef instead of the arrayrefs you are expecting. Try using Data::Dumper to dump out %hoa before you try to access its content. Can you post the code that you are using to build %hoa?

perl -e 'split//,q{john hurl, pest caretaker}and(map{print @_[$_]}(joi
+n(q{},map{sprintf(qq{%010u},$_)}(2**2*307*4993,5*101*641*5261,7*59*79
+*36997,13*17*71*45131,3**2*67*89*167*181))=~/\d{2}/g));'
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