$cat input.txt | ./anWithArgs.pl letters # should spit out lines with a letter
Can't open letters: Datei oder Verzeichnis nicht gefunden.
$cat input.txt | ./anWithArgs.pl numbers # should spit out lines with a number
Can't open numbers: Datei oder Verzeichnis nicht gefunden.
$cat anWithArgs.pl
#!/usr/bin/perl -an

my $type = shift or die "no type";

if ( $type eq 'letters' ) {
  print if $_ =~ /\w/;
} elsif ( $type eq 'numbers' ) {
  print if $_ =~ /\d/;
} else {
  print "bad type: $type"
}
$cat input.txt
a
b
c
d
e
f
g
1
2
3
4
5
6
7
8
9