# With Perl's -s switch
$ ./anWithArgs.pl -type=letters input.txt
$ cat anWithArgs.pl
#!/usr/bin/perl -sn

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