$ cat t.pl
#!/usr/bin/perl
use strict;
use warnings;

# Function to accept only 4 digit numbers
my $v_4dig = gen_validator('^\d{4}$');

# Function to accept only 3 digit numbers
my $v_3dig = gen_validator('^\d{3}$');

# Function to accept only lower-case alphabetic strings
my $v_alpha = gen_validator('^[a-z]+$');

for my $t ('apple', 123, 456, 7890) {
    print "$t:\t", $v_4dig->($t), "\t", $v_3dig->($t), "\t", $v_alpha->($t), "\n";
}

sub gen_validator {

    my $regex = shift;

    # Create function to validate against current regex
    return sub {
        my $data = shift;
        return 1 if ! defined $data;
        if (ref $data eq '') {
            return 1 if $data =~ /$regex/;
        }
        return 0;
    }
}

$ perl t.pl
apple:  0       0       1
123:    0       1       0
456:    0       1       0
7890:   1       0       0