That will replace a / with a \, but only one. Quite likely you need to replace many. The /g switch does that, but there is another trick it is worth knowing (many actually, but we'll focus on just this one). Consider:

my $doc = 'This /and/ that.';

$doc =~ s!/!\\!g;
print $doc;
[download]

Prints:

This \and\ that.
[download]

Note the use of ! to delimit the regex parts to avoid the picket fence effect you get using the default / and needing to quote stuff with \.

For more regex goodness see perlretut and perlre.