my $string = "This is a only a test";
my %words; 
my $i = 1;

# Assign the order of the words as a key, the word itself as 
# a value
$words{$i++} = $_ foreach(split(' ', $string));

# Delete the word that you want to delete
my $deleted_word = delete($words{3});

# Put the word back in
$words{3} = $deleted_word;

# Join the values back together based on the order of 
# the keys
$string = join(" ", map { $_ = $words{$_} } sort keys %words);
print $string . "\n";