The difference is in my use of quotemeta, \Q inside the regex. His version uses your string as a regex, mine turns it into a literal search.

Here is a simple demo for what's going on:

my $string = 'c:\\Windows';
my $search = '\\W';
my $qm = quotemeta($search);
my $raw = $string; $raw=~ s/$search/#/;
my $cooked = $string; $cooked =~ s/\Q$search/#/;
print <<"TEST"
original string: '$string'
search string: '$search'
search string after quotemeta: '$qm'
substitution without \\Q: '$raw'
substitution with \\Q: '$cooked'
TEST
[download]

Result:

original string: 'c:\Windows'
search string: '\W'
search string after quotemeta: '\\W'
substitution without \Q: 'c#\Windows'
substitution with \Q: 'c:#indows'

As you can see, with quotemeta, it replaced the substring backslash+"W", because that's what /\\W/ searches for. Without it, it just searched for the first non-word character and replaced it — that happens to be a ":".