#!/usr/bin/perl

our $type ||= shift or die "no type";

while( <> ) {
  if ( $type eq 'letters' ) {
    print if $_ =~ /[[:alpha:]]/;
  } elsif ( $type eq 'numbers' ) {
    print if $_ =~ /\d/;
  } else {
    print "bad type: $type"
  }
}