#! perl
use strict;
use warnings;

my $file   = 'config.txt';
my $prefix = 'ge-3/1/0.';

my ($router, $max_x, $max_y, $max_z) = get_values($file);

my $max = $max_z // $max_y // $max_x // 0;

set_interface($router, $prefix . $_) for 1 .. $max;

sub get_values
{
    ...
}

sub set_interface
{
    ...
}