Instead of where you use "last;" You could use "return $input;" on that line or use the example below.
I wrote this replacement code

sub input
{
  my $input;
  print "ILLEGAL INPUT $input\n"
    while ( chomp($input = <STDIN>) and $input !~ /^\d+$/);
  return $input;
}
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I changed it slightly in that it will only return input if $input is a number. Your example would return "spam9" as good input.