#!/usr/bin/perl -w
use strict;

my @words = ('These','are','words',"\n");

print "Original: @words";

safesub(@words);

print "After safe sub: @words";

unsafesub(@words);

print "After unsafe sub: @words";

exit(0);

sub safesub {
    print "safesub to uppercase: ";
    my @uppercasewords = @_;
    for (@uppercasewords) {
        tr/a-z/A-Z/;
    }
    print "@uppercasewords";
}

sub unsafesub {
    print "unsafesub to uppercase: ";
    for (@_) {
        tr/a-z/A-Z/;
    }
    print "@_";
}

# output

Original: These are words
safesub to uppercase: THESE ARE WORDS
After safe sub: These are words
unsafesub to uppercase: THESE ARE WORDS
After unsafe sub: THESE ARE WORDS