#!/usr/bin/perl -T

use strict;

%ENV = (
    PATH => '/usr/bin'
);

open my $filehandle, "< :utf8", "test.bin" or die $!;
my $word = readline $filehandle;

my ($untainted) = $word =~ /^(\w+)$/;

if ($untainted) {
    # It passed the regex, so it is "safe".
    system "echo $untainted";
}