The -a switch splits the current line on whitespace and puts the results in the @F array but you are not using the @F array in your code so you probably don't need the -a switch. A simple method of providing switches to your program is Perl's -s switch (described in the perlrun document.)

# 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"
}
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