As more experienced programmers pointed out (thank you to afoken!!!), I've had the buffering issue.
After reading this article at perl.plover.com/FAQs/Buffering.html, I've applied the filehandle HOT. Now my script works as expected and looks like this:
#!/usr/bin/perl -w
use strict;
use warnings;
print "Please, enter the hex number : \n";
S|=1;
my $hex = <STDIN>;
print "$hex", "\n";
chomp ($hex);
print "You've entered hex: ", $hex, "\n";
print "The decimal equivalent is ", hex "$hex", "\n";
Thank you very much for the help!
