most languages (that I know) use the + for both

Yes, but other languages don't have Perl's notion of context. In Perl, the + operator supplies numeric context, but the concatenation operator supplies string context. The difference can be important.

  my $barcode_prefix = "25728"; 
  for my $b (@series) {
    my $chkd = checkdigit($barcode_prefix . $b);
  }
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If you change the concatenation operator there to +, a completely different thing happens and the check digit comes out wrong. (And yes, I have written code that calculates barcode check digits in Perl5.)