Using the autoflush buffer variable trick and the trinary operator along with modifiers and replacing the while loop by using % gives:

use warnings;
use strict;

print checkUPC('064200115896');

sub checkUPC {
    # grab and immediately split upc into array, 1 char per element
    my @chars = split //, shift;
    
    return "should be 12 digits in length" if @chars != 12;
    
    local $| = 0;
    my $sum = 0;
    
    # loop through to sum even and adjusted odd chars
    $sum += $chars[$_] * (--$| ? 3 : 1) foreach 0..10;
    
    # calculate correct check digit
    my $check = (10 - $sum % 10) % 10;
    
    # return error message if wrong check digit was initially given
    return $check != $chars[11] ? "invalid checkdigit...should be $che
+ck" : 'Ok';
}
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Prints:

Ok
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Note that this version returns Ok on success. Replace 'Ok' with undef for original behaviour.